Add deprecated method redirects

[joshbuker]

Glorified note-to-self

Add deprecated methods that redirect to the new method names, e.g.

# @deprecated Please use {#digest} instead
def encrypt(password)
  warn "[DEPRECATION] `encrypt` is deprecated.  Please use `digest` instead."
  digest(password)
end

To be removed in version 1.1.0 following Rails convention of removing deprecated methods on the next minor patch after a major release.

ActiveSupport::Deprecation may be a preferred way to do this, worth looking into at least:
https://stackoverflow.com/a/17339794