VOSpace working directory

Adds the ability to specify a working directory in VOSpace.  For example:

> ex: "python validate.py cands vos:drusk/OSSOS/astromdir"

Working on the local file system remains the same.  However, when working in VOSpace, progress is managed and saved using properties of the VOSpace nodes (instead of sentinel files).  For example, when a file has been processed, instead of creating a <filename>.DONE file, the <OSSOS_TAG_URI_BASE>#done property is set to "TRUE".  A similar process is used for recording indices of sources processed as well as the lock holder.

Please try this out carefully at first and see if you run into any troubles.

Closes #10.