Rename henry symbol Hr to H

In physics, and electronics, the henry (symbol H) is the SI derived unit of inductance.
For more visit: http://en.wikipedia.org/wiki/Henry_(unit)
Add new prefixes for ampere, henry, ohm and volt

This commit adds new prefixes and implicit conversions for:
 * ampere: nA, µA, mA
 * henry: nH, µH, mH
 * ohm: nΩ, µΩ, mΩ, kΩ, MΩ
 * volt: nV, µV, mV, kV
Add short symbol K for kelvin
Merge branch 'mkiedys-kelvin'
Merged branch mkiedys-AHΩV. Version bump to 0.2.1.