add DEntity.id (Closes #3)
avoid alloc in DEntity.touch()

datomic.Entity.touch() returns `this` but has the side effect of touching all the attributes
avoid empty Entity check

an entity will surely never be empty, it will always have at least an id
DEntity.get & .getAs only catch EntityKeyNotFoundException

only turn EntityKeyNotFoundException into None, and let other exceptions pass through
Merge pull request #7 from pellucidanalytics/topic/DEntity

clean up in DEntity