added coordinate_frame reading in from_wcs
bugfix in error reporting
apparently one of my commits restored a previous error
'GLON'->b'GLON' when comparing to wcs
an attempt to parse radesys into coordinateframe.  I think it makes
sense, but I may be mising edge cases
Hide the ``items`` attribute since it doesn't get checked for standard-compliance.
Merge pull request #14 from astrofrog/hide-items

Make ``items`` hidden
If specs version is changed, update list of available attributes, and warn if any of them were previously set.
Added full specs tests and fixed a few uncovered bugs.
Added test of warnings when switching metadata version
Removed debug statement
Skip the two bytes that indicate the packet length for variable-length packets when searching for \xff, otherwise run into issues when the two bytes for the length include \xff.
Fix bug with parsing certain JPEG files
Merge pull request #13 from astrofrog/update-attributes-with-specs

When specs version is changed, update available attributes
Merge pull request #11 from keflavich/fromwcs_coordframe

added coordinate_frame reading in from_wcs
Fixing bugs with to_wcs/from_wcs
Fix failing tests
Added tests for from_wcs/to_wcs
PEP8
Fix failing tests