Fix Mode Change (Arm/Disarm)

[jwillaz]

Add caching of modes so they are not retrieved each time a change is called (reduces mode change from ~10 seconds to 3-4 seconds). Add checks for 'None' instances and make mode change generally more resilient.

[coveralls]

Coverage increased (+0.2%) to 81.117% when pulling d323112 on jwillaz:mode_change_fixes into 2d8b535 on tchellomello:master.

[tchellomello]

Great idea on caching the data. I think part of the performance problem I had on HA 0.55 was related to that.

[tchellomello]

Just address the lint issues and it's ready for merge.

[coveralls]

Coverage increased (+0.2%) to 81.117% when pulling 8cd1fef on jwillaz:mode_change_fixes into 2d8b535 on tchellomello:master.

[tchellomello]

Thanks for your contribution @jwillaz

[broox]

Another way to do this may be to simply use the cached_property decorator. This may be useful for several other properties here as well...

https://github.com/pydanny/cached-property