update vagrant documentation
only check for vagrant-omnibus and vagrant-berkshelf if explicitly using base_image vagrant machine
Rework the Json serialization code

- clarify how a transcoder works, align methods
- Have only a few transcoders, use Guice binder to bind for different types
- clarify IdTranscoder, use standard valueOf method to instantiate
- remove all the static code serializations
- remove most direct ObjectMapper uses, use transcoders everywhere
- remove SingularityJsonObject, move helpers into JsonHelpers
- add general SingularityTranscoderException instead of
  SingularityJsonException (it would be great if that were a checked
  exception or a subclass of IOException but the amount of code
  changes required for that is so painful that we need to let that go
  for now).
Update to Dropwizard 0.7.1
Merge branch 'transcoder-fixes' into hs_staging
Merge branch 'update-dw071' into hs_staging