Fork pypyscrypt for libsodium implementation
Use salsa20_8 from libsodium
Use struct.pack instead of a structure
Remove now unused code
Benchmark libsodium implementation
Wire in the libsodium implementation

Currently 4-5x the speed of pure python on CPython
Replace lost type checks
Fix test description
Make benchmarks executable
Docstrings expansion and cleanup
Merge branch 'use-libsodium'

Use libsodium for the salsa20/8 core in scrypt on cPython when available.
This is roughly 4x as fast as the fallback pure Python implementation.

On pypy this is slower, so the pure python version will be used.