Add 'off' exposure mode

[waveform80]

As of 1.6, there'll be a discrepancy between the way we handle awb_mode and exposure_mode, and their corresponding "manual" attributes: awb_gains, and shutter_speed:

When awb_mode is 'off', awb_gains manually controls the auto white balance. When awb_mode is anything else, awb_gains reports the gains calculated by the automatic algorithm.

By contrast, when shutter_speed is 0, shutter speed is determined by the algorithm named by exposure_mode. When shutter_speed is anything else, it manually determines the shutter speed.

In 2.0 we should make things consistent: add a "fake" value to EXPOSURE_MODES titled 'off'. When exposure_mode is set to 'off', the shutter_speed property controls the shutter speed used. When exposure_mode is anything else, shutter_speed simply reports the shutter speed used (which exposure_speed currently does). In other words, change _get_shutter_speed to use the implementation of _get_exposure_speed, leave _set_shutter_speed alone, and tweak _set_exposure_mode to set underlying shutter speed to 0 when a value other than 'off' is specified.

[waveform80]

Ahh ... apparently there's already an 'off' exposure mode which disables AGC, but I excluded it from picamera because it only appeared to output black when activated. Turns out shutter speed needs to be set explicitly prior to setting exposure mode to off to avoid this. So, this can be added to 1.7 without much issue.