Extruder Calibration Tool V2

This is the next iteration in the design of an [extruder calibration tool](http://www.thingiverse.com/thing:109601) I use to help with the calibration of the extruder on the Felix Printer. I was frustrated with inaccuracy of marking filament and trying to measure with my callipers.

##Updated in this Version:
- Fixed, the Top piece was not sitting flush with the Z axis in the original STL file, as a result unnecessary support would be printed under it.
- Fixed, 1.75mm fiiment now presses snuggly into the top piece and differences in filament sizes should be accounted for i.e. one size fits all.
- Redesigned in Alibre, source files added.
- Added separate STL files for top and base.

##What this thing does:
This helps by providing two flat surfaces that the vernier callipers can easily measure between. It is simple to install and remove without having to remove any filament from the extruder. Just push the filament into the top piece and start the calibration process as