Refactored code into CLI and module for exports

Code that produces final CSS is still in CLI. This needs to be further
refactored either to return a string, or use streams.
Improved error/exit handing
writeMediaRule now generateMediaRule & returns string
Refactor of actual CSS generation
Further refactor of CLI process into module

createMediaRules() handles most business, black boxing q away and
returning method to node callback pattern.

Some other small changes to variables.
Fixed test
generateRuleSet now accepts callback

Several other small refactors for function parameter checking and
callback calling.
Updated README