Add default calibration to fix #20

Added a default calibration for the base motor set to 0 * q.count / q.mm.
Move *Calibration to concert.devices.base

My reasoning here is this: People can't remember things. So the less modules to
remember, the better. concert.devices.base is also a good place as it clearly
states that it ought to be used by all devices and is one of the core principles
on what we build.
Fix documentation
Add unit test to cover the interface
Merge pull request #24 from ufo-kit/default-calibration

Default calibration