Context implemenation and some test cases

[sameercaresu]

This is the implementation of pgettext and npgettext mentioned in https://github.com/zaach/jsxgettext/issues/95.

I have added some test cases for pgettext and npgettext. Other tests, that I have not modified, are passing the tests as well.

Please let me know if you have any comments.

[sameercaresu]

I have added implementation of dgettext, dngettext, dpgettext and dnpgettext. If domain is used it is ignored during extraction.

[ArmorDarks]

Wow, that's nice! Thanks!

[sameercaresu]

Any chance this could go into next release? Thanks.

[sameercaresu]

Hi I have made the requested changes. Please let me know if there's more changes needed.