Ellis Bot V2 - CoreXY

*** COMMENTS WELCOME ***

After designing V1 (https://www.thingiverse.com/thing:2553656/), I got some feed back over at Reddit (https://www.reddit.com/r/3Dprinting/comments/72tvfv/ellis_bot_corexy_request_for_comments/). So with that feed back, this is what I have come up with.

### Design Goals:
Minimum maxium speed of 100 mm/s (150 mm/s would be great, 300 would be awesome) with decent precission and accuracy.
300x300x500 build volume.
100 Micron resolution.

### Changes since V1:
* Replaced most of the printed parts with metal tubing, 2 1/2" x 2 1/2" x 0.125", 2" x 4" x 0.125" and 2" x 2" x 0.125 (gantry part), and metal plates. Now it's more metal than your grandmas Norwegian black metal band.
* Replaced pulleys with F608zz's.
* Single Z axis motor, 3 acme screw design with 2 linear rail guides.
* Should fit on a 24" deep desk (Pretty standard depth)
* Shrunk X and Y down to 300 mm (Well it's a tiny more, but 12" build surfaces will adhere). 500 mm on the Z still.