webhttp storage can get json documents by calling http GET method. for jio allDocs,
the storage will get a file called _document_list which should contain the list
of document id that can be downloaded.

replicatedopmltree storage can convert a list of opml storage (each opml can contain
one or more sub_storage) into an indexeddb. All document in opml tree will have an
entry into the indexeddb storage, the full document is added as attachment in the
indexeddb.

opml and rss feed are splitted into two differents jio storages

/reviewed-on nexedi/jio!51

/reviewed-on nexedi/jio!50

/reviewed-on !48

Fix JIO crash when running inside Safari because of non-existing DOMError object

/reviewed-on nexedi/jio!47

Attachments are synchronized only of explicitely activated from the jIO configuration.
Replication deletes a document only if it doesn't contain any attachment.

Thanks to Cedric Leninivin for his work on this topic.

/reviewed-on nexedi/jio!42

It allow to check recursively if a storage support bulk.

`jIO.util.stringify` was raising if the value of an object was undefined, this changes fix it and provides the same behaviour as the standard `JSON.stringify`

/reviewed-on nexedi/jio!41

@seb, please review

/reviewed-on nexedi/jio!37

/reviewed-on nexedi/jio!35

This helps understanding why browser refuses to open the DB.

Example: Firefox prevent using IndexedDB if browser history is disabled.

/reviewed-on nexedi/jio!33

Fix ReplicateStorage to reduce conflicts during synchronisation.

Creation and modification process now use the same code

/reviewed-on nexedi/jio!29

/reviewed-on nexedi/jio!28

Do not display error in browser not implementing WebSQL.
WebSQLStorage will raise an error is used in such browsers.

JSON.stringify is not stable by design and can not be used to generate the hash.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify

This commit adds a stable version of JSON.stringify, available at jIO.util.stringify.
Relying on the nearly same string representation will simplify compatibility.

Tests were failing because some Firefox versions cannot
parse a specific date-string. So the Date constructor
that receives only integers was used. But there is one
details: this contructor uses months from 0 to 11, so if,
for example, we want to have a date in January, we send 0
as month and not 1.

Fix ERP5Storage to reduce conflicts during synchronisation.
ERP5Storage.get return a new parent_relative_url property which will lead to changes from ReplicateStorage point of view.

Fix QueryStorage/MemoryStorage bugs.

Copy all documents/attachments set in the storage, to prevent unwanting external modifications