Snapshotting for Journal.IO journal (closes #97)

- incl. support for upper replay limit
- refactorings
  * packages util and serialization for journal-commons
  * generalize snapshot storage on local filesystem
Update snapshot section in user guide

- to reflect snapshotting support of Journal.IO journal
Merge pull request #99 from eligosource/wip-97-snapshotting-journalio

Snapshotting for Journal.IO journal (closes #97)