Added support for custom run styles (MGWT-351)

See also:

http://jira.codehaus.org/browse/MGWT-351
https://groups.google.com/d/topic/google-web-toolkit-contributors/BkFHsmjSOLM/discussion

Example usage:

	<plugin>
		<groupId>org.codehaus.mojo</groupId>
		<artifactId>gwt-maven-plugin</artifactId>
		<version>2.5.0-1-SNAPSHOT</version>
		<executions>
			<execution>
				<goals>
					<goal>test</goal>
				</goals>
			</execution>
		</executions>
		<configuration>
			<mode>custom</mode>
			<customClass>com.github.neothemachine.gwt.junit.RunStylePhantomJS</customClass>
			<productionMode>true</productionMode>
		</configuration>
	</plugin>

And if the run style needs an argument, you supply it with `<customArgument>...</customArgument>`.