(bug 43291) statementview can display and add references now

NOTE: Looks and works much better when used together (rebased onto) Ic205d640. Basically, this
      solves the issue of weird entity labels after reloading page after adding references.

References can now be displayed with the jQuerz.wikibase.statementview. Statements are displayed
as jQuery.wikibase.referenceview which for now inherits from jQuery.wikibaseclaimview. This is
because as of now we do not have qualifiers in the claimview or multiple snaks in the
referenceview, so both are very similar. Later the code should be refactored for both widgets to
use a common base widget (which has the task to display and manage several snakview widgets). For
now this solution saves some time.
Furthermore, a new, more generic list widget, jQuery.wikibase.listview, has been introduced. This
contains a lot of ugly code borrowed from the claimlistview. Over time, all list view widgets
should be refactored to share more code, as we require further list widgets for displaying a
Statement's qualifiers and a Reference's collection of Snaks. At the moment toolbar and API related
code is not encapsulated enough, this has not improved in this patch set, it is rather worse now.

The following will be done in follow ups:
- code cleanup
- CSS styling
- minor fixes required, e.g. references counter does not yet update when adding reference

- patch set 3: proper revision store handling when adding references
- patch set 4: 'add' button changed to 'add source'
- patch set 8: fixed optional parameter in repoApi
- patch set 9: rebased

Change-Id: Ib615348067a7bbd2270b6dab128bc53f8f46fcd8