Changed test description for gettext constant
implemented context aware translations as described in rubenv/angular-gettext#53 to use with gettext msgctxt

base functionality on catalog, added attribute translate-context to
directive and handled possibility of extra argument in the translate
filter.

Tests added for filter catalog and directive.
identation fixed for readability
using angular test functions to test for arrays and objects and null to
ignore arguments
filter now accepts an object that can have translateN translatePlural
and translateContext properties and calls the appropriate catalog method

This change allows more flexibility to accept functionality like #104
space to separate angular expression delimiters from filter argument
object
Changed the code that retrieves a translation given a context.

Previously the format of the catalog for translations with context was
[{ctxA: t1, ctxB: t2}, {ctxA: t1_plural, ctxB: t2_plural}] which was not
very clever (duplicate info, bad structure, sorry!).

Now the format is the more concise [{ctxA: [t1, t1_plural], ctxB: [t2, t2_plural]}].

The wrapping arrays in the context object values are mandatory even if
only one element is present (no plurals).

The external array wrapping is optional but allowed to achieve
consistency with how things were already done before context
translations were allowed.
Merge context support (#103).

No plural filter support yet.