Fix/flip longitude where possible. Explain where it is not.

This is in response to #67. Unfortunately there is no easy fix for the inability to flip the RA/longitude axis when using matplotlib projections. The only current fix seems to require the Basemap Toolkit, which we don't want to require.
"highly non-trivial" -> "not trivial" or "highly difficult"?  

Not a big deal if you're attached to it, but for some reason "highly non-trivial" sounded weird to me.