feat(translateService): missingTranslationHandler receives language

Currently the missingTranslationHandler is only invoked with the
missing translationId. For completeness it is important to pass the
full context, namely the language this translation is missing for.

This aids developers in decoupling their missingTranslationHandlerFactory
from the $translate service.
Merge pull request #76 from jamesandres/missing-translate-handler

feat(translateService): missingTranslationHandler receives language
tests(midway): adds some midway testing
update version
Update README.md
fix(tests): travis CI
Load fallbackLanguage

separate load and uses. Load fallbackLanguage if needed
Add tests
Use angular.isFunction
Throw error if no key is specified
Throwing test

Expect load() to throw an error if no language key is specified
Add some tests

test that we can load languages without using them and that
fallbackLanguage and preferredLanguage are automatically loaded
Test fallbackLanguage
Change deferred arg on load

Change ```$uses``` for ```key``` since we are not using the language
anymore
Merge branch 'async-fallback' of https://github.com/sgara/angular-translate into async-fallback