Update build config

- Upgrade to 2.11.1 (but add a cross-compilation target for 2.10.4)
- Upgrade to scalatest 2.2.0
Oops. Need to upgrade to latest sbt for last commit to work
Add Readable typeclass stuff directly in the json lib

1) This can be an alternative way to define how a class should be serialized.

n.b. This is a work in progress. Will create a type class for serializing as well.
Clean up Codec code
Add error handling and automatic instance derivation

No more need for reflection or ObjectMapper
Fix minor typo bug
Add error handling
Remove dependency on Twitter util
Get rid of annoying compiler warnings