I started working on context function pgettext

[buma]

I needed context support so I added it. Since library supports everything else.

My addition is a little hackish IMHO I added third item in list which is returned from getTranslatable. So normal gettext returns item, plural returns 2 item array, context 3 item. First item is key second is context.

Any suggestions for improvement are welcome. I'm not at home in Javascript I think I changed all the necessary code. I'll add some handlebar tests.

Working on #94. Extracting works. I also added tests. I updated same tests that adding ngettext changed.

Joining works if I parse both files (they are correctly deduplicated). But If I join with existing messages.pot like join test is doing they are joined incorrectly.

In addition to correctly joined msgid there is another from the second file