add Makefile and ./install script

The Makefile has build, install and clean targets. The build target just
makes the GitHub.wdgt directory by copying everything to it. The install
target puts it into ~/Library/Widgets and launches it with open. The
clean target removes it.

The ./install script is just the commands from the build and install
targets, added because not everyone has XCode installed.
add install instructions

Add install instructions in README.md for Github.