Get-mac

Added
* Addition of a terminal command: `get-mac`
* Ability to run as a module from the command line: `python -m getmac`

Changed
* `arp_request` argument was renamed to `network_request`
* Updated docstring
* Slight reduction in the size of getmac.py

Dev
* Overhauled the README
* Moved tests into their own folder
* Added Python 3.7 to list of supported snakes

* Python 2.6 compatibility

* Fixed some addresses returning without colons
* Added more rigorous checks on addresses before returning them

* Remove print statements and other debugging output

* Initial pre-alpha