Move to simplejson from json

The stdlib json module handles deserialization of bools differently in Python 2.7 and 3.4. Moving to simplejson to get consistent behavior across all versions.
Updates to consul.api

- Move to property methods for returning the internal attributes of Session for each sub-object type to provide clear documentation and prevent re-assignment on accident.
- Update docstrings quite a bit
- Refactor getting an item from KV to reduce duplicate code
- Add KV.get_record
Add simplejson to the dev requirements
Start building out documentation
Start of unit tests for consulate.api.KV