Refactor and drop py2 support (#49)

* move is_off property to SmartDevice, implement is_on for bulb and use it

* refactor by moving smartbulb and smartplug to their own classes

* drop python2 compatibility, make flake8 happy

* travis: remove 2.7, add 3.6

.travis.yml

@@ -1,9 +1,9 @@
 sudo: false
 language: python
 python:
-  - "2.7"
   - "3.4"
   - "3.5"
+  - "3.6"
 install: pip install tox-travis
 script: tox