#83 RateHelper for compounding swaps (basis swaps)

[Andre Miemiec]

This code implements a RateHelper for basis swaps, which use compounding coupons. For this purpose the existing code from experimental/coupons/subperiodcoupons was taken but significantly changed. All common spread types are supported.
The code was tested against a production environment and concluded with a perfect match.

[Luigi Ballabio]

May you submit this as a pull request on https://github.com/lballabio/QuantLib?