Add toString() method to MessageNano.

- All of the real work for printing the proto is actually done in
  MessageNanoPrinter.
- Uses reflection to find proto-defined fields and prints those.
- Prints all fields, even defaults and nulls.
- Also added a simple test to make sure it handles all proto types well.
  Tried not to make the test too brittle (but hey it's testing a toString()
  so how flexible can it be)

Change-Id: I3e360ef8b0561041e010c1f3445ec45ecdcd2559