Enable translation of plural forms

Issue #238
Remove TranslationStringFactory

TranslationFacotry is only useful for non-plural forms. It only defines a
default domain for a particular string, and it's easily replacable with
a `setdefault`

https://github.com/Pylons/translationstring/blob/master/translationstring/__init__.py#L131

Issue #238
Update l10n source files
Consistant function names
Merge pull request #297 from dodobas/l10n_plural

Support L10n plural forms