Added some documentation about drivers.

Specificaly, the drivers package now has documentation about the standard
interface that a driver must implement. As noted, the error handling behavior
needs to be decided on and documented.
Reworded documentation to avoid implementation details.

The bit about no-ops isn't something the user should care about - the
functions just shouldn't be used at all.
Merge pull request #59 from CCI-MOC/driver-docs

Driver docs