add support for extra sync methods (fixes #124)

- http://docs.datomic.com/javadoc/datomic/Connection.html#syncExcise(long)
- http://docs.datomic.com/javadoc/datomic/Connection.html#syncIndex(long)
- http://docs.datomic.com/javadoc/datomic/Connection.html#syncSchema(long)
- tidy up javadoc