Multiple journal implementations

- LevelDB based journal
- Journal.IO based journal
- in-memory journal

Closes #2
Update example code
Journal optimizations (#11)

- scalable memory consumption
- output message deletion
Batch replay (#11)
Batch replay in replicator (closes #11)
Update link to milestone 1 features and enhancements
Sequence-structured LevelDB journal (closes #13)

- Encapsulate key management
- Generic OutputWriteMsgCache
- LeveldbJournal code reorganization
- Move key serialization code to location where it is used
- Document journal creation
Travis configuration
Travis configuration update
Include build status into documentation
added plugin for idea project files generation
Link to new API docs and working branch
Event sourcing based on stackable modifications

- closes #16
- fixes #18
 API changes and enhancements

- responder API changes
- emitter API changes
- multicast can now also process non-event messages
- support behavior changes for concrete receivers
- call unhandled() if target receiver doesn't handle message
- fix races in composite recovery spec
- rename Publish to Emit
Fix race in FsmExample by making destination idempotent

- experimental Idempotent stackable modification (#14)
Idempotent now acks retained messages
API documentation
API documentation enhancements
API documentation
Drop obsolete user documentation