chore(features/modeling): adjust default append distance
feat(core/Canvas): add parent to add* / remove* methods

If users pass a parent element to any of the add* or  remove* methods,
the Canvas will maintain the parent <-> child relationship between the
elements.

Related to bpmn-io/bpmn-js#6
chore(project): release v0.3.0
feat(overlay): add overlay api

A service that provides an API that allows to add HTML overlays to the diagram.

See #54