Greg's Wade extruder for Prusa i3 redux

I like misan's modified Greg's for Prusa i3 (and Bob is my uncle), but I wanted a few small changes, starting with not having the head of the idler hinge bolt hit the X carriage (the newer one, as shown in the photo). Here's a list, in no particular order, of the changes I've made:

* Set base extra depth to 2mm so the head of the idler hinge bolt doesn't interfere with the X carriage.
* Added a small thumb-grip to the top of the idler.
* Increased hole for 608 bearing from 22.3 to 23mm b