feat(on): Add support for events on bpmn elements

- add an interface to add an event (such as 'click') on a rendered bpmn element

related to CAM-719
fix(Transformer.js): support webservers that provide correct MIME types

Transformer.js expects to get the BPMN XML as string, which works with
servers that deliver *.bpmn files as application/octet-stream, e.g.
Node.js, but fails on servers that correctly deliver them as text/xml.
In the latter case the BPMN XML will be already provided as a DOM
Document.
improve(getting started experience): improve readme and demo
Merge branch 'master' of github.com:camunda/camunda-bpmn.js

Conflicts:
	src/bpmn/Bpmn.js
Merge pull request #5 from falko/render-url-fix

Fix renderUrl to work with webservers that provide correct MIME types
fix(domClass): remove domClass reference
feat(renderer): add SVG export