Expose a hook on the login function

Some organization I work with uses Gigya because they are not allowed to store users email address directly into their database.

The alternative approach is to use gigya for auth and then store a salted,hashed representation of the email address in the database. We can hook easily before the creation of the user is made into the database and change the email address but it is currently impossible to hook before the module look for existing user in the db with the non-salted/hashed version of the email.

I've included a simple module_invoke_all that provide a simple hook.