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