Replace UTF character in README.rst

setup.py failed on my Arch Linux Raspi using Python 3.3 with the following message:

    [nils@raspi RPIO]$ python setup.py 
    Traceback (most recent call last):
      File "setup.py", line 24, in <module>
        long_description=read('README.rst'),
      File "setup.py", line 6, in read
        return open(os.path.join(os.path.dirname(__file__), fname)).read()
      File "/usr/lib/python3.3/encodings/ascii.py", line 26, in decode
        return codecs.ascii_decode(input, self.errors)[0]
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 77: ordinal not in range(128)

I could trace the error back to the line

    * PWM via DMA (up to 1µs resolution)

In `README.rst`: The `µ` can't be properly translated to ASCII. This Pull-Request replaces it with a simple `u`.