Enhancement: Add proper Error and 404 page

Instead of displaying exceptions a custom error and 404 page are displayed.
Bugfix: Fix user registration.

registrationDate and avatarSource are properly initialized because table has a
Not Null constraint
Dependency: javax.mail and Freemarker added

* Dependency for javax.mail was added.
* Dependency for Freemarker template engeine added.
Bugfix: Throw DuplicateRegistrationException only if Registration is valid

This avoids that duplicate error message on the registration jsp page.
Bugfix: Avoid caching of login, registration, passwort-forgot and password-reset
Bugfix: Custom message for password validator
Feature: Support for sending mails added

A MailService interface was defined for sending mails using the freemarker template
engine. Furthermore two functions for sending password-reset link and
 password changed mail was added.
Enhancement: Better Design for the login and registration page

The login and registration page have been enhanced with a better design using
bootstrap 3.
Feature: Password-reset functionality was added

Users can now reset the password. This is done by specifying the email address.
An email containing a reset link (unique token) will be sent to the users email
address. The token is valid for 24 hours. Using the link the user can reset their
password.
Merge branch 'signup'