Updated contribution guidelines

We sadly still need to be more clear that tickets need to adhere to certain standards to be manageable...
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
Merge branch 'devel' of https://github.com/foosel/OctoPrint into mrbeam
* added a svg to gcode plugin (copied from cura-plugin, so there is probably still a lot of unused code in it)
* added 'svg' as a supported model file extension / format
* added a new dialog for customizing gcode conversion of svgs