Added conversions between degrees and radians
Implemented units checking very badly

Overrode is_a? to check against Units.

For example:
Units.meters.is_a?(:meter) will return true
Moved NumericMixin to its own file

A small bit of cleanup that's been bugging me for awhile
Moved the monkey patching of Numeric to a file

This is rather trivial, but it's also slightly prettier.
Short-circuit method_missing when no block was given

Units.initialize doesn't take blocks, so if a block was passed, the call must be for something else.
Fixed the travis CI image links to reflect the new repository name
Added support for the exponent operator
Minor cleanups