Now using watchdog to monitor a "watchdog" folder for gcode or stl files being added

Use this to define a folder on your system from which to automatically import/move added files into OctoPrint.

Also now monitoring the uploads folder so that if files are removed externally during runtime, the metadata.yaml will be cleaned up.
Forgot to add new dependency on python watchdog
And also forgot to push one more file

Kids, never code while on a moving train with a flaky internet connection, you tend to miss important things...
Renamed folder "watchdog" to "watched"
Bug fix for Emergency Stop
Merge pull request #473 from Salandora/EmergencyBug

Emergency Stop should not only trigger an event but also at least cancel the print

Closes #448