Fixed $MONITOR_PORT detection, thanks choffmann
Really fix $MONITOR_PORT detection

Fixes issue #215
Added quotes around greps/cats to help alleviate possible problems
with spaces in directory/file-names.

Removed $TARGETS as it was an unsafe wildcard and was only used
in "make clean" which itself has been simplified to just delete
the $OBJDIR rather than waste time deleting individual files.
Build core objects in subdirectory, fixes issue #82
Add "Advanced Usage" and Bare-Arduino-Project reference in the README.md