Improve the chances of Robohydra finding its system-wide plugin directory.

Robohydra isn't always installed in /usr/local/share or /usr/share. For instance, on a Mac, with node.js and npm installed via Homebrew, Robohydra may be installed in `~/node_modules/robohydra`

Searching for systemwide plugins relative to `lib/robohydra.js` location improves the chances of finding them no matter how robohydra is installed.