implemented simple fits reader.
added unit support and checking. the reader can now read the simplest of fits_files (crval1, cdelt1, ..).
some smaller fixes for the wcs reading/writing
Merge branch 'master' into fits_readers
added the fits_capable_wcs list (checking for `.from/to_fits_header`)
Merge branch 'master' into fits_readers
Merge pull request #38 from wkerzendorf/fits_readers

Fits readers