Second pass at generalized WCS.
Minor renames
Fix naming of separable to join
Add arithmetic operators
Rename series to concatenate.  Add more organization to transform docs.
Fix typo.
Add "axis" type to describe the frames to which the axes belong.
Change author order
Address @nden's comments
Fix typo reported by @migueldvb
Merge pull request #14 from mdboom/gwcs

Second draft of generalized WCS standard.