* added new json hidden widget
* default input should not be hidden. Users can define their own template for that
* removed unused template; * added resources field for snippet vars
* implemented functionality for parsing and using admin resources
* fixed bugs with parsing resources; widgets media are now rendered
* implemented functionality to expose global settings in template.
Due to security issues django settings should not get exposed in
templates (secret credentials might get stolen). What will get
exposed in snippets settings will be up to the developer's decision.
* widget resources are now added to form media.
LUN-1960: implemented display for predefined widgets
LUN-1960: implemented js lib for snippet widgets registration inside plugin forms
* fixed js custom exception + added variables getter utility
* hide predefined widgets when not available
* don't render predefined vars unless they have widget or resources
* filter predefined vars to make sure they have data defined
LUN-1960: added events for snippet widgets lib
* added helper template tags
* added helper for snippet widgets js lib
* add the ability to initialize/validate list of variables.
* added required snippet js lib to model admin
* pep8 fix
Merge pull request #48 from pbs/LUN-1960

LUN 1960: custom admin snippet widgets