ObjectMapper reuse & config

Enables ObjectMapper reuse through implicit injection. ObjectMapper can
be accessed & configured.

Additionally, added a per-request method which allows overriding the
ObjectMapper for a specific request
def->var, ensuring mapper isn't re-instantiated
Indirection: Abstracting Jackson out to allow other impls e.g. json4s
No more implicit magic, adding layer of indirection.
DefaultJacksonSerializer is now an object

Also, serializer in the controller is a settable var for setting the
default serializer without making a new render method.
AppService: Removing Jackson Serializer
Import cleanup
Merge remote-tracking branch 'twitter/master' into mapper-reuse

Conflicts:
	src/main/scala/com/twitter/finatra/Controller.scala
	src/main/scala/com/twitter/finatra/ResponseBuilder.scala
Update JsonSerializer.scala

Newline removal
JsonSerializer: Removing author tags
Merge pull request #126 from Xorlev/mapper-reuse

ObjectMapper reuse & config