Workflow API: add tool version and parameters for each step when showing a workflow.

Otherwise to get this information an extra API call to download the JSON
representation, followed by a tricky mapping of the step ids are necessary.
Add documentation to workflows API create().
Workflows API: really allow import with installed_repository_file by not crashing if workflow_id, ds_map or history are not specified.

Reorder code so that a new history is created after checking the sanity checks.
Merged in nsoranzo/galaxy-central (pull request #341)

Workflows API enhancements (pull request #337 corrected)