cqrs.commandHandler: add extension methof for getting a value or default from a dictionary
cqrs.commandHandler: hide buildAction method from the public
cqrs.commandHandler: return a dictonary for CommandHandlers property
cqrs.commandHandler: extract interface from commandHandlerRegistry
cqrs.commandHandler: rename local variables to better distinguish them
cqrs.commandHandler: add nsubstitute to test project
cqrs.commandHandler: add tests for multiple handlers and no handlers
cqrs.commandHandler: fix default value is list.empty rather than null
cqrs.commandHandler: refactor container init
cqrs.slides: add slide for the commandHandler demo
cqrs.legacy: add demo slide for legacy version, add new demo methods
cqrs.slides: add notes for legacy demo
cqrs.slides: emalgamate query and command slides