Deprecate FlowMachine::Factory

[jhanggi]

Deprecate the methods in FlowMachine::Factory in favor of having them straight on FlowMachine

Example:
Previous: FlowMachine::Factory.workflow_for(...)
New: FlowMachine.workflow_for(...)

And add notes about these methods to the README.

[ahorner]

Just pitching an idea: these factory methods could be moved directly onto Workflow. FlowMachine::Workflow.for(...) reads nicely (essentially an alternate constructor), and has the side effect of keeping the logic for constructing workflows encapsulated in the class itself, giving you more freedom to make internal API changes.