Changeset [ae05d908b04dd15dfe6de3f98921faa193f72b60] by Robert Buels

make track metadata source urls just be relative to the base
(i.e. index.html), add support for a "class" param that specifies what backend class to try to use, and make the dojo.require not be called
if the class is already loaded, and not throw if it doesn't succeed.
https://github.com/GMOD/jbrowse/commit/ae05d908b04dd15dfe6de3f98921...