have function to clear the `.mvnrepository` directory if its too full

[jstrachan]

related to the check function here
#246

when starting a build, we should check the %age of disk free. If its, say, less than 20% free we should clear the disk (as its really only a cache anyway).

So we need 2 %age numbers; when to start warning the user its too full; and when to clear on startup.

Maybe we warn at <= 30% free or less and clear at <= 20% as a rough starting point?