Adds $(OUTPUT) files to `make clean` target

Modifies the `make clean` target so that output binaries
bin/gemma and/or bin/gemmaf will be removed so that they
will definitely be rebuilt after `make clean`
Merge branch 'master' into fix-make-clean-bins

Conflicts:
	Makefile
Merge pull request #11 from wtsi-hgi/fix-make-clean-bins

Adds $(OUTPUT) files to `make clean` target (fixes #8)