Merge pull request #496 from edx/feature/cdodge/add-a-mixed-modulestore

Add a 'MixedModuleStore' which aggregates between XML and Mongo Modulestores
Merge branch 'master' of github.com:edx/edx-platform
add a try catch around the staticstorage lookup, which can throw an exception
Merge pull request #723 from edx/fix/cdodge/bad-links-can-cause-exception-on-rewrite

Fix/cdodge/bad links can cause exception on rewrite