Add configurable control over the classpath.

[aknrdureegaesr]

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.

Configuration example:

<configuration>
    ...
    <classpath>
        <entry>target/test-classes</entry>
    </classpath>
</configuration>

This fixes #5 .

Please pull, and thank you for providing fine software.

[ljnelson]

Hi, thanks for your input. Let me look at this (it's been a while). Offhand I don't see any issues.