Stepsperunit parameters

[felipesanches]

This is what I consider a better way of configuring axis scaling and extruder parameters.

I have left the hard-coded magic numbers because I still dont know which specific parameters where used to obtain those values. Please consider expressing all those configurations by defining the following attributes, so that they are easier to tweak/understand:

• steps_per_revolution
• microstepping
• belt_pitch
• pulley_teeth
• extruder_gears_ratio
• bolt_diameter
• z_thread_distance