Direct Drive Extruder

A direct drive extruder housing and tension hinge.
Inspiration has come from many direct drive extruders found here, amongst which:
* [Byteborg's Direct Drive Extruder](http://www.thingiverse.com/thing:127873) (big big fan of this)
* [Compact Bowden Extruder](http://www.thingiverse.com/thing:275593)
* [One Piece Simple Direct Drive Extruder](http://www.thingiverse.com/thing:43098)
* [mariusmartin's direct drive extruder](http://www.thingiverse.com/thing:379166)