Fixed an embarrassing merge error from waaay back. Didn't disrupt operation but caused funny error messages in the log.

Thanks to @nophead for reporting this.

Closes #433
Create CONTRIBUTING.md
Bug fix for Emergency Stop
Merge pull request #475 from Salandora/master

Bug fix for Emergency Stop
Insert src folder at first position of sys.path
About time to declare this as a stable version, should have done this ages
ago...
Merge branch 'master' into staging

Conflicts:
	setup.py
	src/octoprint/server/ajax/control.py
Fixed a couple of issue with the gcode viewer

Most notably the interfering options tracked in #398, but also made "Center viewport on model" and "Zoom in on model" automatically deselect and de-apply if viewport gets manipulated by user, therefore properly allowing the re-select by the user to work to.

Closes #398
(cherry picked from commit f195af0)