Refactored to allow pluggable async execution.
Refactored to allow pluggable async execution.

Moved subprocess.Popen in LocalRunner class.  Class can be replaced
with alternative execution logic.  

Also changed logic to poll job status.  Required changing multiple methods
to accommodate a status of "started".   Did quick sanity testing manually with 1.wdl and seems to work. 
broadinstitute/wdl,openwdl/wdl,openwdl/wdl,broadinstitute/wdl,broadinstitute/wdl,broadinstitute/wdl,openwdl/wdl,openwdl/wdl