feat(rest) Added bulk commands for process instances

Conflicts:
	engine-rest/src/main/java/org/camunda/bpm/engine/rest/ProcessInstanceRestService.java
	engine-rest/src/main/java/org/camunda/bpm/engine/rest/impl/ProcessInstanceRestServiceImpl.java
	engine-rest/src/test/java/org/camunda/bpm/engine/rest/helper/MockProvider.java