Implemented DELETE and GET /mac_address_ranges/<uuid>

* Added exceptions.MacAddressRangeNotFound,
  exceptions.MacAddressRangeInUse
* Implemented tests for plugin.get_mac_address_range
* Implemented tests for plugin.delete_mac_address_range
Merge pull request #84 from asadoughi/delete_mac_address_range

Implemented DELETE and GET /mac_address_ranges/<uuid>