No wheels sliders for C-Bot and derivatives

Update 9/11/2016

I have added new sliders for Z and Y axes. In addition, I have added a new carriage based on croadfeldt's Hot End generator (source code for the changes is available on [github](https://github.com/spiffcow/r-bot).

I no longer recommend using PLA. Taulman 645 or Taulman bridge is optimal for these parts.


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This is a PLA slider to use in place