E3D Titan Extruder for Sintron i3

My i3 is kind of a nightmare hybrid, with the X gantry taken from Sintron i3 patterns.

Changes from GyroBot's design:
* Moved the belt clamp features and belt pass-through so they were in the same plane as my idler bearings and spindle. I thickened the back of the carriage by 3mm, and subtracted 3mm from the fan.
* Since I'm using 5x10x4 idler bearings on my X axis and a 20-tooth 2GT pinion, I moved the belt pass-through channel a little bit to avoid rubbing.
* Pulled the fan mount holes