Added toFieldWithContext to Codec
Add test demonstrating use of codec.as with a coproduct type
Update docs for Discriminator and Discriminated
Fixes #35

Adds CoproductCodecBuilder#as for deferred xmapping based on a Generic
instance.
Merge branch 'master' into topic/shapeless-2.1

Conflicts:
	src/main/scala/scodec/Codec.scala