feat(variable): Add variable to process instance

- Now it is possible to add a new variable to process instance.
- Therefore a dialog exists, which can be triggered via a button in the
  toolbar inside the process instance view.

- Implements a new directive called 'variable', which handles the input
  fields to add/edit a variable

- null variables can be added too now

related to CAM-1082, CAM-1115
Merge branch 'master' of github.com:camunda/camunda-bpm-platform

Conflicts:
	webapps/camunda-webapp/webapp/src/main/webapp/app/cockpit/pages/process-instance.html