Remove "rm" dependency

Remove dependency on rm. exec("rm") works on windows if you include git/bin in your PATH, but many users select the default value when installing git, which means they only get git.exe and none of the linux helper executables. A pure node solution is better anyway, I think :)

This will add a node dependency on the "wrench" library, which is what I used in the crx module. 

I also added the system dependencies from the crx package, "openssl" and "ssh-keygen", into the README. If either of these are not installed the grunt task gives very cryptic errors.