feat(translateService): extracts default interpolation in standalone service
feat(messageformat-interpolation): implements usage of messageformat
feat(translateService): implements usage of different interpolation services
feat(translateFilter): teaches filter to use custom interpolation
feat(translateDirective): teaches directives to use custom interpolation
feat(translateService): informs interpolator when locale has changed

- this commit also teaches interpolators to temporarly use fallback language
if needed
Merge pull request #90 from PascalPrecht/extract-engine

Pulls out interpolation logic into services