Adding sideload REST API for TCGA data along with one sample file. Rest
URL is same as upload, a post request to /dataset; however instead of
'upload' parameter it'll now look for the 'load' parameter with the
value being one of the string keys in WorkspaceController.presets map
Fixed TSV mime type, import cleanup
Front end for loading preset datasets
Merge pull request #452 from lev-kuznetsov/master

Loading preset datasets