support Entity.db method (fixes #122)

add in method that returns the database value that is the basis for an entity
http://docs.datomic.com/javadoc/datomic/Entity.html#db()
ensure that all impure methods have () (fixes #123)

methods such as Connection.database are not referentially transparent
add support for Database.invoke (fixes #121)

add invoke method with two overloads, one for an entity ident and one for an entity id
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
Merge pull request #126 from dwhjames/topic/db-invoke

add support for Database.invoke (fixes #121)
Merge pull request #127 from dwhjames/topic/entity-db

support Entity.db method (fixes #122)
Merge pull request #129 from dwhjames/topic/unit-methods

ensure that all impure methods have () (fixes #123)
Merge pull request #131 from dwhjames/topic/sync-methods

add support for extra sync methods (fixes #124)