This commit puts all import related stuff into the import module.
The core module remains as an entry point to require the basic modules
needed by bpmn-js. At the time this is { import, draw }.
This commit adds
* handling of bpmn update as command listeners
* label support
Related to bpmn-io/diagram-js#45
BREAKING CHANGE:
* rename bpmnModeling -> modeling to achive parity with diagram-js