Retrieve last few printed gcode commands through API

[peteruithoven]

To visualize what the printer is doing applications need access to the gcode. Because the webserver on the box can't handle sending the whole file at once it would be interesting to log the last few executed gcode lines. This could be a printer/gcodetail endpoint, with arguments like num_lines (which should be limited).

I'm not sure whether this will be possible for the Makerbot, since we convert this to the makerbot format when we retrieve it through the api.

[woutgg]

We could implement the limited gcode retention into the GcodeBuffer. For various reasons, we should modify the makerbot driver to use that class as well, which would then automatically add this feature.