Teach Secret#to_s and initializer to allow use to re-hydrate a new Secret.

Secret#to_s now returns a URL safe Base64 encoded String which wraps
a Base 36 encoded version of the internal OpenSSL::BN #secret.

This String can also be passed to Secret.new(STRING) to
re-hydrate a new String that is internally identical to
the original Secret.

Removed no longer used (and funky) Secret#to_base64 method.