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
Event Progress bugfix
Autodetect Baudrate bugfix
And another place I forgot to adapt to the new sd file list format
Always send commands with checksum when printing
API: Allow cross-origin requests

When a REST API is called from within a browser page from a different domain,
the server needs to explicitly allow this. This makes the OctoPrint
API available also to other webapps, for instance those hosted on same network in hackerspace.
API: Only allow cross-origin requests if explicitly enabled

To enable, set the key "allowCrossOrigin" under "api" in config.yml
No user interface for this option yet
API: Add documentation for newly added cross-origin config option
Added checkbox to the settings dialog for CORS, also slight changes in the documentation
API: Clarify Access-Control-Request-Header handling

'headers' was a way too generic name, and the code structure implied
it was used in place of 'resp.headers' or 'request.headers', which is not the case.
Also add comment about the purpose of this code.
ACCESS_CONTROL_REQUEST_HEADERS => Access-Control-Request-Headers

While flask internally translates that just right back, this makes the implementation more readable since it matches the standard header name
Merge branch 'jonnor/api-allow-cors' into devel
Forgot to git add image newly added to documentation
Documented SockJS interface, deprecated /api/state
Renamed a file
Insert src folder at first position of sys.path
Insert src folder at first position of sys.path