Document crypto_provider in README, and help w/SCrypt confusion

This is a solution to solve the confusion from the scrypt change in 3.4.0:

https://github.com/binarylogic/authlogic/pull/417
https://github.com/binarylogic/authlogic/issues/411
https://github.com/binarylogic/authlogic/issues/416

~~It outputs an explanation for the missing scrypt gem for newcomers.~~

~~Other alternative: to actually require scrypt in the gemspec~~

The branch adds `scrypt` as a gem dependency. 

It also removes a hack I added before for autoloading providers for when the gem wasn't installed (now it will always be).