auth against sha512-crypt password hashes, upgrade to pbkdf2

This code is meant to facilitate migration of GNU Social sites to Pleroma. GS uses SHA512-crypt for its password hash. Therefore this code implements SHA512-crypt validation of passwords, and upon successful validation, upgrades the hash to pbkf2 in the database. Includes test. Caveat: it uses a GitHub library, which creates a dependency on a C compiler. Additional caveat: I don't know what I'm doing.