Rename scmtools.crypto to scmtools.crypto_utils

On case-insensitive file systems, the import of 'Crypto' inside
scmtools.crypto will fail. This problem also appears when attempting to
run Review Board inside a Linux Virtual Machine, where the code has been
shared from a Mac OSX Host. Since the default OSX filesystem is
case-insensitive, case-preserving, the import will fail as well.

The import now succeeds after the patch, and the unit tests exercising this code now pass.