Raw support for heatmap and documentation

References #88

Heatmap GET request now supports a raw parameter. If given this as true
then the data returned will have its secondsWorked field be the actual
seconds worked and not a scaled value. This is for information purposes
primarily and could be used to generate reports about the number of
people working and that like.

Added documentation to API specification detailing the use of the raw
argument that can now be passed to the heatmap endpoint.
Added list of errors returned by API

References: #88

Added list of error messages to the specification so that clients can
handle all errors gracefully and be able to check the message of the
various errors in order to display whatever dialog neccesary to inform
the user that there was a problem.

A reminder to myself for later, when performing the next release of the
API I should update the version number of the API within both the yaml
file and the returned JSON from the get request api.py.