don't require sqlalchemy; synchronize sqlite-only code with sqlalchemy code;
fix drop method for sqlite-only code; add docs to suggest sqlalchemy install


git-svn-id: svn+ssh://svn.mystic.cacr.caltech.edu/pathos/klepto@559 8bfda07e-5b16-0410-ab1d-fd04ec2748df
Merge branch 'master' of https://github.com/uqfoundation/klepto