Autoload CryptoProvider classes

In a recent commit (https://github.com/binarylogic/authlogic/commit/c2eae1ce2e30a8c613313cfe167cb46b0fff0287) I removed some old require code for bcrypt/scrypt. 

This ends up throwing the `LoadError` in your app if you don't have those two gems in your Gemfile, but we still don't necessarily need the gems to run Authlogic. 

This change autoloads each `CryptoProvider` when necessary rather than requiring all of them when loading Authlogic.