Audit/History plugin for NHibernate
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 * Operates on tables rather than object graphs.
   * History tables have no foreign keys between them.
   * Collection relations which are backed by an explicit table become a list of datestamped relations.
 * All audited entity types MUST use NHib's versioning feature (optimistic concurrency). If Guid versions are desired instead of integers, implement an IUserVersionType.
 * All audited entity types must have a corresponding 'audit entry' class which implements IAuditHistory.
   * Audit entries cannot reference entities or each other, and cannot have collection-typed properties. They are simple tuples of value types.
 * An IAuditEntryFactory must be provided to AuditConfigurer. Given an auditable entity, this must create an instance of the appropriate audit entry class and copy all audited data to it.
   * AutoMapper is useful for this.
 * Audit entry classes must be persistence-mapped by the consuming application. There is no automatic mapping functionality yet.
 * There is no internal support for soft deletion. This is an application-level detail rather than a persistence concern. Deleted entities will still have audit history, but cannot be fetched or undeleted.
   * If you want soft deletion, add an IsDeleted boolean property and Update the entity instead of Delete-ing it. Possibly add an event listener to ban the use of Session.Delete against that entity type.
 * Audit datestamps must be stored and serialised to millisecond accuracy. This means that SQL Server's 'datetime' type cannot be used for audit tables, and datestamps can never be roundtripped through the Javascript Date type.
   * There is a Serialisation.AuditDatestampFormat constant which defines a date format sufficient for retaining fidelity when serialising as a string.

Audit Entry Structure 
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For Entity like:
 * Id Guid Primary Key
 * VersionId Int32 Version
 * <properties...>
 
Define an EntityAuditHistory like:
 * AuditId Int64 Primary Key
 * Id Guid
 * VersionId Int32
 * PreviousVersionId Int32
 * AuditDatestamp DateTimeOffset
 * AuditedOperation Int32 (enum:AuditedOperation)
 * <properties...>


Interval Trees
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Intended for use in 'current' data or denormalised copies of history data. There is no intrinsic support for adding these to history entities since updating becomes problematic.