Add support for $translate.instant()

Thank you for maintaining grunt-angular-translate. I was missing the $translate.instant() support (also requested in #18). Here we go!

I could not run 'grunt test', but 'grunt clean i18nextract && nodeunit test/angular-translate_test.js' worked for me. What do I miss to get 'grunt test' running? I installed all packages via 'npm install' and installed nodeunit via 'npm install nodeunit -g'