Merge pull request #78 from nsoranzo/travis

Enable Travis CI testing
fixed a problem that may be triggered by running "python setup.py test"
Add Travis CI and the pypi download count icons to the README
docs: fixed nonlocal image problem
Merge pull request #80 from simleo/fix_nonlocal_img_3

Fix nonlocal image problem
Slightly more descriptive README
Docfixes for workflow run's replacement_params argument.
replacement_params docs bit moved to the original API
Fix history import_dataset() when run after a tool.

A history.refresh() before

  old_ids = set(history.dataset_ids)

was needed, but the code can be simplified now.
Remove from ObjHistoryClient 1 method which should be called from History instances (objects cleanup).