Added support for the collection initializer enabling more convenient syntax

var sf = new SagaFixture(...)
sf.AddSagaData(new SagaData());
becomes:
new SagaFixture(){ new SagaData()}
Also expose AvailableSagaData as IEnumerable on the class (partly to support the collection initializer pattern, and maybe avoid some demeter)
Merge pull request #175 from STRUDSO/sagafixture

Added support for collection initializer enabling more convenient syntax