Fix circular import (#44)

To avoid the circular import between `IborIndex` and `Libor`, I've moved the `from_name` class method to a separate file. This fixes #44.

@phenaff As this reverts a change that you made, could you let me know if I've overlooked anything, or if you have another implementation in mind? As long as `IborIndex` does not import any of its derived classes, this should be fine.