remove credentials memoization.
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
Merge pull request #189 from larssono/develop

remove credentials memoization.
fix a merge mistake