Update to newer Rna2D.js

The newer library fixes problems in showing motif boxes when dealing with
missing nts. In addition, it requires underscore.js so I've added this to the
js/ directory.
Merge branch 'dev' of github.com:BGSU-RNA/RNA-3D-Hub into dev
New bug fix in rna2d
Add length, basepairs, and bp/nt to popups