add make clean target

removes build/release/ and build/temp/ dirs
Readme: mention make clean
do not remove build/temp build/release by default

reuse the files at re-compilation. only changed .o will be compiled.
use make clean   to start fresh and compile all -- eg when errors occur.
rename to clean-> distclean, fix
Merge branch 'master' into make_clean

Conflicts:
	README.md
Merge pull request #65 from breznak/make_clean

Make clean & keeping built files