Unify all four 1.3 branches

This unifies all four 1.3 release branches by 
  - merging back 1.3_2.11 and ensuring that cross-building works
  - some binary compatible refactoring (just moving code around to other files) to centralize the fundamental shapeless version dependent parts
  - creating new `spray-routing-shapeless2` and `spray-routing-shapeless2-tests` projects that mostly share sources with the old shapeless1 variant

Having just one branch will simplify publishing a lot and makes it much easier to keep everything in sync.