Implementation of SSI Mighty mini pump control

Allows control of Mighty Mini pump with pumpy:

 * Added optional stopbits argument to Chain constructor. Allows one stopbit to be used for Mighty Mini pump.
 * MightyMini class implements all Pump functions for this pump. `setdiameter`, `withdraw`, `settargetvolume` and `waituntiltarget` are no-ops as they cannot be implemented for this pump.
 * New commandline switch `-MightyMini` to enable this pump.