Added bit type

Commonly used for network bandwidths (that's what I need it for anyways)

Hope the code isn't too ugly, feel free to improve it/suggest improvements - I'm a ruby newbie

quick example
```
irb(main):003:0* Filesize.from('100mbit').to('MB')
=> 12.5
irb(main):004:0> Filesize.from('12.5MB').to('MBit')
=> 100.0
```