make pysftp an optional dependency
fixed cache bugs when downloading to a second downloadLocation
merged from upstream
added ALPHA status for tables to comments
Merge pull request #188 from cbare/develop

made pysftp an optional requirement and fixed caching bug
Merge branch 'develop' of github.com:Sage-Bionetworks/synapsePythonClient into develop

Conflicts:
	synapseclient/client.py