ACT-1418 Unit tests and suggested fix for NPE on uncaught error event

Work done at Hackergarten Vienna together with @rafacm and
@florianpirchner
Upgrade xerces dependency for cxf (QA failing due to Xml parser changes of yesterday)
Added process diagram documentation to userguide
Merge branch 'master' of https://github.com/Activiti/Activiti
Removed security from diagram rest service
Changes for OSGi package definition
ACT-1581: Refactor ProcessDiagramGenerator to take BpmnModel as input instead of ProcessDefinitionEntity + ACT-1580: Add method to retrieve BpmnModel (Pojo version of BPMN 2.0 process) to RepositoryService
Merge branch 'master' of github.com:Activiti/Activiti
Fix for test failing due to xml parser refactoring
Fix distro build
Merge branch 'master' of https://github.com/Activiti/Activiti
Added unit tests + fixes for feedback step type of simple workflow
Added form key to historic task instance
ACT-1580: added simple unit test
Fix for history form key storage
ACT-1583: Json for feedback step
Merge branch 'master' of github.com:Activiti/Activiti
ACT-1577: Index ACT_IDX_HI_ACT_INST_PROCINST should use EXECUTION_ID_ instead of PROC_INST_ID_
Adding logic/logging for randomly-faling ServiceTaskSpringDelegationTest
Fix for ACT-1586: ExecutionQuery returns wrong results when using multi instance on a receive task