Added PicoJSON.

Review: https://reviews.apache.org/r/18699
JSON parsing and equality.

Review: https://reviews.apache.org/r/18700
JSON to protobuf parsing.

Review: https://reviews.apache.org/r/18701
Added a 'bool' to the test protobuf message.

Review: https://reviews.apache.org/r/18724
Include picojson when building libprocess.

Review: https://reviews.apache.org/r/18702
Include picojson when building Mesos.

Review: https://reviews.apache.org/r/18703
Made visitors be top-level structs.

When compiling with GCC 4.2 we need to keep Boost visitors as
top-level classes instead of nest them in functions.