Added JSON serialization/deserialization.

* pandoc-types now requires GHC generics, hence ghc >= 7.2
* Text.Pandoc.Definition now derives ToJSON and FromJSON instances
  for pandoc types
* Text.Pandoc.JSON module exports ToJSONFilter typeclass (a replacement
  for the ToJsonFilter exported by Text.Pandoc, which will soon become
  a deprecated synonym).