Add support for count/exists queries.

This pull request adds support for both exists and count.

```
// Get the number of rows that match the where-clause
final int count = new Select()
                    .from(Entity.class)
                    .where("...")
                    .count();

// Get a boolean value that indicates if any rows exist that match the where-clause
final boolean exists = new Select()
                    .from(Entity.class)
                    .where("...")
                    .exists();
```