Added initial support for Android plurals.
1° attempt to support weblate plurals handling.
Merge remote-tracking branch 'upstream/master' (temporary merge)

Conflicts:
	translate/storage/aresource.py
3 way merge: Merged local velo's changes, master changes and plurals handling changes.
- Re-applied Android characters escaping to resources with XML markup
- Fixed generalized functions used to read and write resource contents (used for string and plural tags)
Fixed Android character escaping when saving resources with HTML mark-up.
- Improved characters escaping for text with HTML mark-up (UnitTest complaint)
- Improved characters un-escaping for text with HTML mark-up (UnitTest complaint)
- Added a couple of tests for escaping mixed with HTML mark-up
- Added some Android plurals unit test
- Fixed some error in Android plurals handling
- Handled translatable attribute also for plurals
- Handled weblate way to pass plurals (as a "list" class instead of a "multistring" class)