Rename libsodium pwhash functions from xsalsa to salsa

This change looks to be done in libsodium 0.5.1, because scrypt doesn't
actually use xsalsa. Using both here to support 0.5, but also future versions
if they drop the xsalsa fallbacks.
Merge branch 'xsalsa-to-salsa' into libsodium-next

The libsodium git pwhash function has been renamed
Use the low-level scrypt interface from libsodium when available
Test all libsodium implementations
Merge pull request #7 from jvarho/libsodium-next

Update to libsodium 0.6 support
Fixes #6