Play Framework Integration

[wolfendale]

There should be a separate library (it can be part of this repository) which gives some useful features for integration with Play! since this is how we're intending to use the library.

Feature suggestions:

• A public interface which is idiomatic in Play.
• Conversions from play-json to our internal expression Value ADT.
• Helper to integrating with Play's i18n functionality
• Helper to access Play's reverse routing
• Helper to access useful properties of a Request object (Lang etc)

[chrisjameswright]

Raised #44 for the conversions from play-json, but haven't done anything yet to work out where that fits in terms of the overall interface