Document for the REST API

See https://docs.google.com/a/openquake.org/spreadsheet/ccc?key=0AgmeiGIi49FLdGU2Nnp0ejJZcUxUM0hmWjhiS1pHWWc#gid=0

This adds documentation for the methods implemented thus far. Some methods (such as those to abort/delete calculations) are not yet implemented.