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
Better error reporting for timelapse rendering and system commands, updated pnotify and added possibility to add a "more" section to notifications
Removed accidentally committed dummy error (for testing pnotify migration)
supportRepetierTargetTemp being true is a too broad expression, also check for TargetExtr or TargetBed on line

See #486
Filename might be None, do not try to pass that to os.path.basename.

See #486
Forgot to add some files to git again, argh!
Also support extended sd file list (including size in bytes) from printer

 See also #486 where support for SD_EXTENDED_DIR format was requested between the lines ;)
Forgot to adapt verifyFileExists to new sd file list format
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