chore(Gruntfile): remove browserify bundling

Bundling diagram-js via browserify makes only limited sense as people
would need to bundle it with their extensions / setup anyway.

Removing it to keep the build slim.
fix(layout): correctly crop multi-point connection

Related to bpmn-io/bpmn-js#48
chore(project): slightly increase max allowed code complexity
fix(context-pad): revert context-pad to plain style
feat(navigation): add navigation plug-ins

This commit moves the navigation plug-ins for scrolling, dragging and
touch interaction from bpmn-io/bpmn-js to diagram-js.

Related to bpmn-io/bpmn-js#124
fix(move): drag on left mouse button only

Related to bpmn-io/bpmn-js#124
feat(move): cancel move on ESC

Related to bpmn-io/bpmn-js#124
test(move): do not override global event