Introduced a method to clean build directory

Created by: d3rrial

Do not actually merge this, please. I wrote an e-mail to @eidheim earlier and proposed a method to clean up the build directory. Now this is obviously a hideous proof of concept and whether or not such a cleanup function is actually useful is very debatable. There are no mutexes set or anything, this is not write-proof, I deleted dirty with boost, etc. as I said, proof of concept, please do not merge!

I'm just putting this here, just in case.

PS: Sorry for the whitespace mess, I used juCi++ to edit it and still had "clean up whitespace" enabled in the options and that turned into what you see now.