Allow users to specify a build directory

If a user wants to speed up the build by compiling to a ramdisk, he should be able to set the build directory in ~/.m2/settings.xml, something like this:

<settings>
<profiles>
<profile>
<properties>
<buildDirectory>/tmp/privatebuild/${project.basedir}/${project.version}</buildDirectory>
</properties>
</profile>
</profiles>
</settings>