Support for real 'micro' prefix and fix 'Ohm' symbol support

Mu vs micro issue (#133)

While testing the code I found that:

Is using: GREEK SMALL LETTER MU U+03BC GREEK SMALL LETTER MU μ

But, at least for me, is more common to use: MICRO SIGN U+00B5 MICRO SIGN µ

So I'm using:

REFIX_MICRO = [u"&#x03BC;", u"&#x00B5", "u", "micro"]

Which looks pretty lame, and I hope the cut & paste worked ;-), but includes both: the micro sign and the greek letter.

The ohm detection is wrong:

We always apply lower() so we never really get uppercase omega. This is the correct list:

UNIT_R = ["r", "ohms", "ohm", u'\u03c9']

BTW, I found that an uncommon unicode that we also support is:

'OHM SIGN' (U+2126) SI unit of resistance, named after G. S. Ohm, German physicist preferred representation is U+03A9

The lowercase is the same for:

'GREEK CAPITAL LETTER OMEGA' (U+03A9)

So the above code covers both.