removed users migration, removed user models non-message methods, remove...

Hi! After some time spent in the project, I have some changes that I think would greatly benefit the project.

* Migration: I think the CreateUsersTable is unnecessary. It also created errors on the migration, and I had to manually delete this migration. I don't like the package would force fields, such as "surname", "username", etc, which my app does not use. I am only looking for a messaging system. I think it is up to the individual user to ensure he has a users table anyway.

* Pichkrement\Messenger\Models should not have methods such as "getReminderEmail", "getAuthIdentifier", etc. This created php errors in my app. Also, defiantly not $rules, $fillable, $hidden, etc, as this again has something to do with my own app configuration, and not the messaging part.

* Removed implemnets form User model