Fix ENOENT errors on Windows

On the command line, Windows does some hax where it finds browserify.cmd and coverify.cmd if you just run `$ browserify` or `$ coverify`.  In Node you have to do the hax yourself.  (See: [this StackOverflow answer](http://stackoverflow.com/a/17537559/1188341).)  Also updated the PATH code to be platform-independent.  With these changes, #1 is resolved for reals.