add a class to emulate a file, and only store the part of the file
that was read/accessed...  This reduces storing an 11MB file down
to under 100KB...  It also allows tests to run w/o the whole file...
Put the original files in fixtures/original...

fix up a couple of issues w/ parsing CRW files, and also allow the
ability to skip parts of the CRW file...  This allows skipping
large parts, like the CCD data and the large thumbnail..