Revisit password hashing

[untitaker]

We use rust-crypto's bcrypt implementation directly to hash passwords manually. As mentioned in #39, rust-crypto is not audited in any way, so we should probably look for alternatives.

• NaCl/libsodium/sodiumoxide have utilities to generate password hashes. This requires linking against another C library which the user has to install manually, which is really unfortunate.
• ring only provides pbkdf2, see https://www.reddit.com/r/rust/comments/46s75m/rustcrypto_apparently_abandonned_alternatives/d0952sp

[untitaker]

0abb67d replaces direct use of rust-crypto with libsodium

[untitaker]

Reopening, I've reverted those commits. The dependency situation is even less convenient than I thought (Ubuntu 12.04 doesn't have an appropriate package). See https://github.com/dnaq/sodiumoxide/issues/116

[untitaker]

I have opened #41 which links libsodium statically. The dependency situation is still not pretty.

[untitaker]

Done!