More Robust Filament Guide Bar for Prusa I3 Metal Frame

I like the filament guide bar by tdiggs, but it just wasnt robust enough for me. The end supports kept snapping. So this is a more robust version of the same idea. Improvements:

* Larger end supports with larger diameter connectors which are less likely to snap off.

* The inner surfaces of the guide bar are rounded slightly to allow the filament to move through more smoothly