Implement bit-byte unit equivalency and nibble unit

This is a feature request that was already mentioned here: https://github.com/astropy/astropy/issues/2897#issuecomment-55081397

It would be nice to implement bit-byle unit equivalency
```python
>>> from astropy.units import Quantity
>>> Quantity(1, 'byte').to('bit') # currently gives an error
```

Also @astrofrog mentioned that it would be nice to have the [Nibble](http://en.wikipedia.org/wiki/Nibble) unit (= 4 bits) available.