Extend public interface

One of the conveniences that developers come to expect from Flask extension libraries is the ability to import most functionality through the extension. In the original release of Flask-Simon, any Simon functionality needed (e.g., `Model`) had to be imported from Simon directly. These new imports allow most things to be imported directly through Flask-Simon.

Closes #1
Update examples

The examples are being updated to reflect the public API changes in 3df71ab.