feat(spring): add support for spring process apps

- add SpringProcessApplication and SpringServletProcessApplication for
  supporting process application bootstrapping using spring.
  #CAM-446

- add support for defining a managed process engine inside a spring
  process application context xml file. The process enigne bean must be
  constructed using the new
  org.camunda.bpm.engine.spring.container.ManagedProcessEngineFactoryBean
  #CAM-445

- add support for Spring Expression Resolving in process applications
  through
  org.camunda.bpm.engine.spring.application.SpringProcessApplicationElResolver
  #CAM-444