README: move to reStructuredText formatting

* Moves README file to reST formatting.  (addresses #5)

* Amends location of workflow figures as per changes on the upstream
  yawlfoundation.org web site.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
README: addition of travis, coverage, pypi badges

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>