API: system hook request functions and documentation updated

Refactor network graph

Added system hooks link to doc index

* updated system hooks documentation and code comments
* fixed access to system hooks if no user given resulting in a `500 Server Error`
* added tests

Fix API return codes

Conflicts:
	doc/api/projects.md
	spec/requests/api/projects_spec.rb

Capistrano deploy example

Autofocus to username input (LDAP login).

* Network::Commit
** Removing unnecessary accessors.
** Removing add_refs methods.
* Network::Graph
** Removing unnecessary accessors.
** The 3 times loop of commits don't need.

Additional Admin APIs

Allow connection to Redis via unix socket

* Module: Graph -> Network
* Class: JsonBuilder -> Graph

Refactor README to split production/development environments

Because model shouldn't know about view logic.

All the info to return codes from the API functions are available in the `README.md` file as suggested.

Allow connection to Redis via unix socket, using
unix:/var/run/redis/redis.sock for example.

Default behaviour does not change, except that the full Redis URL must
be configured, with redis:// for tcp or unix: for unix socket.