Add support for grammar in js message parsing

jqueryMsg parser does not support GRAMMAR and language specific grammar rules are not available at javascript.

Atached patch tries to resolve this:

1. make jqueryMsg understand GRAMMAR. Depend on the patch at Bug 33653
2. Make SITENAME with value mw.config.get( 'wgSiteName' ) available in parser without the need of module users explicitly adding it.
3. mw.language get convertGrammar, can be overridden per language as in php
4. Make global variable wgGrammarForms available at js by adding to ResourceLoader default variables.
5. Language.php get a method to filter wgGrammerForms for the current contentLanguage.
6. Qunit tests

Not committing the code now, because core code is in 'slush'

Version: 1.18.x
Severity: normal

Attached: