docs(guide): fixes syntax error in language negotiation example
fix($translateProvider): determinePreferredLanguage was not chainable

If registerAvailableLanguageKeys() was called without the aliases
parameter determinePreferredLanguage() was not chainable, because it
did not return this. Fix it and cover it with a test case.

Closes #487
docs(guide/en): removes deprecated docs for default interpolation
fix(docs): removes explicit protocol declaration for assets

fixes #513
fix(demo): use `.instant()`