Added extension for Predicate expression

This allows to build dynamic filter.
Added Timestamp component entity
Moved DateTime extension methods to its own class
Added unit tests for DateTimeExtensions
Added base class used to implement persistence tests
Bumped nuget version number
Renamed 'Crux.Domain.Tests' to 'Crux.Domain.Testing'
Added 'Crux.Domain.Testing' to nuge publish task
Gemfile.lock changes
Updated package version
Set NUnit category
Bumped package version
Added Email Tests
Fixed issue where transaction started before unit of work
Bumped up nuget version
Upgraded crux rake to 0.2.0