Allow encryptors other than authlogic_sha512

[tristanm]

Implements :encryptor config option which allows developers to set encryptors other than just authlogic_512. Any encryptor from Devise Encryptable should be able to be used, including custom encryptors, although I have only tested authlogic_512 and bcrypt.

The default has been left as authlogic_512 to remain compatible with existing apps. However, all specs pass when the default is changed to bcrypt.

Specs only check for the inclusion (or not) of Devise::Models::Encryptable and the value of Devise::Models::Encryptable#encryptor. I figure any other functional tests to do with the encryptors belong with the encryptors themselves in Devise. That said, let me know if there's anything else which needs to be tested.

[tristanm]

Travis is failing because mysql2 isn't in Gemfile... anything I should do for this?

[kushniryb]

@tristanm Pease rebase against current master, thanks in advance!