Add configurable control over the classpath.

Add entries to the front of the `-cp` that's handed to the `java` invocation. This is convenient for "stored procedures" aka H2 [user defined functions](http://www.h2database.com/html/features.html#user_defined_functions).

Configuration example:

    <configuration>
        ...
        <classpath>
            <entry>target/test-classes</entry>
        </classpath>
    </configuration>

This fixes https://github.com/ljnelson/h2-maven-plugin/issues/5 .

Please pull, and thank you for providing fine software.