Printrbot Simple X axis Upgrade (30cm print width)

https://www.youtube.com/watch?v=e109tYzCs8A

This work is based on Andy Haer's work : http://www.thingiverse.com/thing:215294/

I've modified his work to fulfill my needs of a heavier glass bed.

What i've changed :

- remodelled the above 3 parts completely (to remove holes and avoid printing problem like in Repeatier)
- extended jaw's length
- changed bed levelling (for levelling from below)
- toughened parts to support glass
- added M3 hex nut knob to tightened bed levelling