Url fix
More help information for 2D diagrams.

I've added some more help on 2D diagrams. We talk about what can be done and
the source of the data.
Add best chains and models, experimental method
Cache motif releases
Redesign Motif Atlas home page
Tweaks to the RNA Structure Atlas homepage
Merge pull request #16 from BGSU-RNA/feature/2d-help

More help information for 2D diagrams.
Keep both changes
Minor tweaks