Change cache directory to match XDG spec

[johnsaigle]

(This is branched from #16 and it should be merged first)

Addresses #2 in which an argument is made for using the XDG Spec instead of using a results directory in the same folder as the script. Both OS X and Linux are supported.

Also changed:

• Added function comments
• Error handling for when the interface is configured wrong
• Rearranging imports according to PEP standard
• Misc readability improvements