- Binary and string support

Implemented binary support in tandem with strings. This meant minor internal changes. Remove some todos.

- Documentation
- Specs
- Tests

This should not break any old functionality I hope!
I think it should raise exception (ie, `function_clause`) instead of returning non-translated string, because `gettexter_server:dpgettext(Domain, Locale, Context, Text)` works properly only `when Context :: binary(), Text :: binary()`