Implementation of pygatt backend

[ChristianKuehnel]

As requested in #50, here is a first implementation of a Backend for Blue Giga Bluetooth adapters based on the pygatt library.

[coveralls]

Coverage decreased (-9.2%) to 76.38% when pulling 3e303d0 on ChristianKuehnel:pygatt into 0af1185 on open-homeautomation:master.

[coveralls]

Coverage decreased (-9.2%) to 76.38% when pulling 9b2d57f on ChristianKuehnel:pygatt into 0af1185 on open-homeautomation:master.

[coveralls]

Coverage decreased (-9.2%) to 76.38% when pulling 9b2d57f on ChristianKuehnel:pygatt into 0af1185 on open-homeautomation:master.

[coveralls]

Coverage decreased (-9.2%) to 76.38% when pulling 9b2d57f on ChristianKuehnel:pygatt into 0af1185 on open-homeautomation:master.

[coveralls]

Coverage decreased (-9.2%) to 76.38% when pulling 9b2d57f on ChristianKuehnel:pygatt into 0af1185 on open-homeautomation:master.

[coveralls]

Coverage increased (+6.5%) to 92.032% when pulling 6fd36b9 on ChristianKuehnel:pygatt into 0af1185 on open-homeautomation:master.

[coveralls]

Coverage increased (+5.9%) to 92.157% when pulling be53999 on ChristianKuehnel:pygatt into fae905b on open-homeautomation:master.