Updated contribution guidelines

We sadly still need to be more clear that tickets need to adhere to certain standards to be manageable...
Fixed gcode analyzing after file upload
Merge pull request #625 from imrahil/gcode_analyzer_fix

Fixed gcode analyzing after file upload
Added changelog entry for bugfix
Keep track of files that are currently being used (file being printed, source and destination for slicing) so that they can't be deleted

Also added logging to exception branches where the exception was formerly just swallowed.
Automatically refresh slicing profiles in slicing dialog when profiles are added or removed via the Cura plugin settings
Properly reselect default slicer and profile in slicing dialog after sending a slicing request
Make "Slicing done" notification green

Implements #558
Don't die if you're not able to log a print (e.g. for prints from SD)
Made "Streaming done" notification green
Generate the key used for session hashing individually for each server instance
Generate the key used for session hashing individually for each server instance
(cherry picked from commit 118a4f7)
Merge branch 'master' into devel

Conflicts:
	src/octoprint/server/__init__.py
	src/octoprint/settings.py
Allow login of the same user from multiple browsers without side effects

So far when logging in from two different browsers, then logging out in one of them the user was logged out across all browsers. This should now be changed in so far as that each individual browser session is tracked and only that session is ended by a logout that belongs to the browser where the logout button was clicked.

Should fix #556
Tornado's StaticFileHandler now supports content streaming, use that instead of our own implementation

Also allow for definition of access validation and serving as attachment though.

Hopefully also fixes #606, since that was possibly called by our own implementation of content streaming, which now has been removed completely.
Enable logging of tornado errors by default, to make sure octoprint.log will contain meaningful entries if something goes wrong in our tornado customization
Generate the salt used for hashing user passwords individually for each server instance
Merge branch 'master' into devel

Conflicts:
	src/octoprint/server/api/__init__.py
Preparing release of 1.1.1
Cura Plugin: Filament diameter and hotend temperature where not correctly extracted from profile and supplied to CuraEngine, resulting in low print quality

(using the default of 2.85mm filament diameter doesn't work well for 1.75mm machines...)