Object-based JSON serialization

[mfine]

Provide full serialization of JSON objects. This change introduces a static from_json method for each message class, and a to_json method for each message class. This enables serializing and deserializing full objects to and from JSON. It's not super optimal on deserialization in that it parses out the JSON and then parses it again through construct, but that can be fixed up later (it wasn't trivial to reconstitute the construct containering). Tests were added around the JSON serialization and deserialization via raw_json outputs in the test YAML. Sub-objects do not have JSON serdes methods.

/cc @mookerji

[swiftnav-jenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
http://zazu.ci.swiftnav.com/job/libsbp_Pull_Requests/8/
Test PASSed.

[mfine]

Fixes #103.

[mookerji]

retest this please

[swiftnav-jenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
http://zazu.ci.swiftnav.com/job/libsbp_Pull_Requests/12/
Test FAILed.

[mfine]

Hmmm, doesn't like the requirements file in tox?

Could not open requirements file: [Errno 2] No such file or directory: 'requirements.txt'
ERROR: could not install deps [-rrequirements.txt, pyyaml]

[mfine]

Screw it, I'll put pyyaml it in the requirements.txt - tox and other stuff is already in there.

[swiftnav-jenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
http://zazu.ci.swiftnav.com/job/libsbp_Pull_Requests/13/
Test FAILed.

[swiftnav-jenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
http://zazu.ci.swiftnav.com/job/libsbp_Pull_Requests/14/
Test FAILed.

[mfine]

retest this please

[swiftnav-jenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
http://zazu.ci.swiftnav.com/job/libsbp_Pull_Requests/16/
Test PASSed.