Working implementation of the login form.

Added in a form interface for logins. The EntryPoint for both the
registration and login forms will be the same. Changing between the
two will involve setting the correct Screen to the container. A few
changes have been made to support this.

* Renamed the RegisterPage class to ContentPage.
* Renamed the container id in register.html.

Login functionality will utilize the Apache Shiro framework for
authentication and jBCrypt for password hashing. As Shiro does
not have support for Bcrypt, I extended the CredentialsMatcher
interface to add support for Bcrypt.