Pulls out interpolation logic into services

this commits introduces new interpolation logic architecture. when merging this in, angular-translate is able to use different interpolations at a time. one is able to set a default interpolation via `$translateProvider.useInterpolation(interpolationServiceName)` or, in case one uses `MessageFormat`, it already comes with a shortcut method `$translateProvider.useMessageFormatInterpolation()`.

its also possible to add more interpolations in case one wants to use angular-translate's default interpolation application wide, but use messageformat interpolation for certain translation ids. in that case, simply add interpolation services via `$translateProvider.addInterpolation(interpolationServiceName)` and angular-translate takes care of the rest. All this was discussed in #15 .