* Changed values for RequestedLifetimeCount and RequestedMaxKeepAliveCount

Also made them changeable via kwargs

* added comment

* reworked so period can be a CreateSubscriptionParameters

Maybe this works better...

* Split a space dump and cache

dumping and caching really serve different purposes; also cache is not
fully implemented and should only be used in special cases

* Fix dead history methods, docs

Fix history methods and add support for historizing a list of nodes
Add doc strings
pep fixes

* Cache fixes, close session when server stops

fix caching of standard address space
try to suport windows cache file
close session on stop so that all subscriptions get removed (need to
serialize nodes)

* Fix cache file check, fix tests

* Improve lazy loading dict

at least support builtins for cache, otherwise features like
node.delete() don't work

* Update high level historize method, update examples

* let caching and dump/load work at same time

experimental commit for discussion
these changes let caching and dump/load work together

* Revert "let caching and dump/load work at same time"

This reverts commit 0fde446e3f2c105aad7aacc34c243414b7accc4e.

* Remove uneeded import

* Update history tests

* make naming more explicit

* Small name refactor, clarified doc strings

* export to xml based on namespaces

* cleanup of ua_utils.get_nodes_of_namespace

* fix export Enum and custom datatype variables

* fixed extensionobject export again (removed typo)

* Test for import/export EnumValues

* move XmlExport.get_variable_basetype to ua_utils.py

* undo cleanup of get_variable_basetype

Can't check 'not val' because variable nodes with a value of 0.0 don't
get exported.

* do not create Variant with value None when they are supported by protocol

* add get_default_value to uatypes. This will be usefull at several places like opcuamodeler

* set correct default value in high level type creation method

* Made subscribe handle most iterable items 

like dicts, etc

* replaced try/except with isinstance

* Improve instantiate when parent has string node id

* Make String node id instantiation more clear

* Fix string to value for boolean

Can't use bool(string); will always return true
Make string to UA ints more readable

* Ignore XML Extensions tag

Other modelers make this XML element which contains a model and hash
from the generation tool

* Add test for instaniate with String NodeIds

* Handle import of Null values

* also export namespace of referenced nodes

* xmlexport: also export ns used in browsename

* __str__ for NodeData

* heavy cleanup of xmlimporter

* more xml import fixes

* fix importing lists of primitives

* export and import array dimensions

* fix by-ref error

* remove debug print

* fix xml test

* do not store ns uri when not necessary

* sdfsdfsd

* Guid as a primitive

UA Guids based on python UUID type

* Guid primitive fixes, import datetime

* Fix Guid NodeId, typos, tests

* Add guid test, python 2.7 fixes

* Guid fixes for modeler

* Pep fixes for edited files

* Fix pep refactor

* Remove string check for datime/guid

this function assumes correct value is supplied, update doc string