feat(engine-rest): add http basic authentication servlet filter

- allows customization of an authentication provider
- switched off by default in the rest.war

related to CAM-822
Merge branch 'master' of github.com:camunda/camunda-bpm-platform

Conflicts:
	engine-rest/pom.xml
	webapps/camunda-webapp/webapp/src/main/java/org/camunda/bpm/webapp/impl/security/filter/Authorization.java
docs(engine-rest): document ProcessEngineProvider interface