Validate input data before adding new games.

Changes to Application.java:
- Added validation on the parameters supplied to the Application#insert() method.
- Populated the DynamicForm object in #insert() with human readable error messages.
- Awaited creation of players until it was sure that the form was valid.

Changes to Player.java:
- Made sure that no players will be created with an empty name.

Seems like the MinLength constraint for player names doesn't apply when using dynamic form. That's why I added the NullPointerException to #getOrCreate(String).

Also updated README.md.
Get rid of the static DynamicForm object (gameForm) in Application.java.