Merge pull request #36 from ajd4096/master

Add support for M82/M83 absolute/relative extruder positions
Merge branch 'master' into devel
Started work on adding extrusion control
Merged devel to master, adjusted README a bit.
Added extrusion controls, kept motor and fan controls in general control area as suggested (recurring demand suggests that they definitely aren't something to be added by anyone individually via custom controls), made custom controls always position themselves below the general stuff.
Merge branch 'devel-richardjm'
Realised I wanted to use the fan control during printing