Mrw support

Support for all the MRW producing cameras from Minolta and Konica Minolta. This code has been tested with and is currently merged into the master version of darktable. It has been tested against example files of all the MRW producing cameras. 

The only cameras not supported by this are a few G-series compacts that had a hidden menu item to enable MRW support. Chances are there are very few of those files in existance and adding support for them is trivial given some example files.