Added new units

Hi Darren,

Thanks for the nice package. I have just started using it to convert units specified by the user in a XML-based description language into defaults expected by the simulators supported by my package.

I wasn't sure of your policy on adding new units into quantities but there were just a few units and scaled units that I needed (eg. M, uF, nF, kOhm, MOhm, etc), so I have added them to what I think are the appropriate modules. They pass all the tests so hopefully they are good to go.

Cheers,

Tom