Add new skipBuild property.

* New skipBuild property for skipping the 'grunt build' portion.
* Allows for fine grained control of execution, tying test and
  packaging pieces to different maven lifecycle phases.

This nicely allows for the following execution setup:

    <executions>
        <execution>
            <id>build</id>
            <phase>prepare-package</phase>
            <goals>
                <goal>build</goal>
            </goals>
            <configuration>
                <skipTests>true</skipTests>
            </configuration>
        </execution>
        <execution>
            <id>test</id>
            <phase>test</phase>
            <goals>
                <goal>build</goal>
            </goals>
            <configuration>
                <skipBuild>true</skipBuild>
            </configuration>
        </execution>
    </executions>

Which allows one, for example, to perform a test without waiting for all the packaging/build work to occur as well.