Add clean target to Makefile

[oxr463]

These invocations of rm -rf $@ for each target in the Makefile should be put in a separate clean target. That way it doesn't remove everything each time the user re-runs make.

This would be especially helpful for all of the large files that are downloaded during the build process.

[oxr463]

A good example would be this: #20

[rgl]

Not sure what you mean. What do you think it's being deleted, why and when?

[oxr463]

One example is this line: https://github.com/rgl/windows-vagrant/blob/master/Makefile#L205

Deleting that directory also deletes the ISO that gets downloaded, which is over 1GB.