new EnumerableHelper.Create overloads and renames

Parameters renamed to resamble Observable from Rx
Removed after and before overload added new ones

The new overloads follow Rx conversion
Added Cycle extension method

Also added missing null check in HasAtLeast
Extensions.Zip is conditiontally extension method

Enumerable.Zip was added in .NET 4.0 - so now the quivalent
Extensions.Zip will be an extension method for .NET 2.0, 3.0 and 3.5
filling the gap.