IOP Exposure - deflicker update

Deflicker modes explanation:
* Mode "automatic" - on export, in commit_params() || process() of exposure iop, calculate exposure compensation using one of two methods (DEFLICKER_MODE_). **WARNING: using this mode, DT can now automagically calculate proper exposure compensation for each processed image without any user interaction - extremely useful for timelapses and video frames**
* DEFLICKER_MODE_THUMBNAIL - this is what been implemented initially. exposure compensation is computed in from histogram of pre-module data
* DEFLICKER_MODE_SOURCEFILE - exposure compensation is computed from histogram of original RAW source file. This is how ML module [post deflick](http://www.magiclantern.fm/forum/index.php?topic=10496.0) is working.