Refactor MCF handling out of pypyscrypt
Implementation using the scrypt python module if available
Fix test decription
Use pyscrypt.py in package import if libscrypt isn't available
Update README to document py-scrypt usage and version
Merge branch 'use-py-scrypt'

Use the py-scrypt bindings if available when libscrypt cannot be found.
This is done in preference to the pure python implementation.