Add 'trueplay' property to SoCo

[pwt]

This PR adds a gettable/settable property trueplay, with accompanying tests. The property allows the Trueplay state of a speaker (True or False) to be determined, and to be changed.

Devices that do not support Trueplay, or which do not have a current Trueplay calibration, will return None on getting the property, and raise a NotSupportedException when setting the property. Can only be set on visible devices. Attempting to set on non-visible devices will raise a SoCoNotVisibleException.

Tested on S1 and S2 devices, devices that don't support Trueplay, and devices that do support Trueplay, with and without a current calibration profile.

[coveralls]

Coverage increased (+0.1%) to 57.431% when pulling 5d2dfdd on pwt:add_trueplay_property into cf5cd34 on SoCo:master.

[coveralls]

Coverage increased (+0.1%) to 57.431% when pulling 75b479e on pwt:add_trueplay_property into cf5cd34 on SoCo:master.

[rytilahti]

Works fine on a trueplay tuned beam, changing the value reflects immediately to the settings in the mobile app. The NotSupportedException gets raised correctly on a non-tuned Symfonisk speaker.

[pwt]

Passes all check jobs locally, and has been independently tested, so I'm going to merge this.