feat(vars): Add variable instance query

- Add to the runtimeService the method 'createVariableInstanceQuery()' to
  create a variable instance query
- Implement the variable instance query
- Add testcases to test variable instance query

related to CAM-941
Merge branch 'master' of github.com:camunda/camunda-bpm-platform

Conflicts:
	engine/src/main/java/org/camunda/bpm/engine/impl/VariableInstanceQueryImpl.java
	engine/src/main/java/org/camunda/bpm/engine/runtime/VariableInstanceQuery.java
	engine/src/test/java/org/camunda/bpm/engine/test/api/runtime/VariableInstanceQueryTest.java