Makefile

Tweaked the makefile, you can now do:
 
````sh
make BIN_DIR=../bin
make test BIN_DIR=../bin
make clean BIN_DIR=../bin
````
..etc and they will behave as expected. Clean will not remove the actual $BIN_DIR in case it's set to an important directory, only the files and directories generated by make in $BIN_DIR (interp, higgs, ..etc) will be removed.

Default behaves as before.