a basic schema manager

manages a schema as an acyclic graph, and tags the nodes of the graph when they are first transacted
a direct port of https://github.com/rkneufeld/conformity/blob/master/src/io/rkn/conformity.clj and https://github.com/Datomic/day-of-datomic/blob/master/src/datomic/samples/schema.clj
Merge pull request #63 from pellucidanalytics/topic/schema-management

A basic schema manager
bump version numbers in preparation for release

bump Datomic version number to 0.8.4020.26
bump specs2 version number to 2.0 and fix compilation
bump Datomisca version number to 0.5
bump version numbers for sample projects
minor cleaning
Merge branch 'develop'

Pull in changes in development branch for 0.5 release
bump version to 0.6-SNAPSHOT
minor: optimize DEntity .keySet and .toMap
fixes #64: add overloads to SchemaEntity.add
remove erroneous implicits

ToDatomicInj[DRef, DRef] simply isn’t needed and is covered in the more general ToDatomic type class
ToDatomic[DRef, DId] breaks the functional property of the ToDatomic as ToDatomic[DId, DId] holds as well, so DId could map to both DId and DRef!
refactoring implicits for writing to a reference attribute

introduce ToDRef type class
improve the PartialAddEntityWriter type class to properly handle writing to reference attributes
add tests for the implicits
optimize To/FromDatomic for DColl
tidy up Attribute2EntityReader implicits
turn temp and final id overloads into type class
remove unneeded AddEntity overloads
minor: rename addRaw to addEDN
minor: use ToDId for DId.apply
Merge pull request #66 from pellucidanalytics/topic/id-overloads

Make overloading of temp and final ids uniform
Merge branch 'maintenance' into topic/implicit-conv

Pull in upstream changes.
minor: seal the ToDRef and DId traits
minor: fix use of DRef (should use DKeyword)
support singleton writes to set-valued ref attributes

the value for a cardinality-many, reference attribute can either be anything convertible to DRef or a traversable collection of anything convertible to DRef