add mmHg in pressure units

[trappitsch]

Since you were open to it in #61, I added mmHg as an additional unit. it is only very slightly different from the definition of Torr.

Reason for adding it I think are:

• widely used still in the medical field
• Every Physics 101 class discusses mmHg because it is one of the most intuitive ways to see pressure
• While for most intent and purposes it is interchangable with Torr, the definition of the two is not exactly the same and so I think it warrants having them separately in here.

[thejpster]

Thanks! I guess we should do a release now.

[trappitsch]

Thanks! A release would be great, I'm using the package for some instrument drivers at the moment - mostly vacuum pump and gauge controllers at the moment.