Topic/plt 430

* Added **CouponIborFxResetDefinition** as an extension of **CouponDefinition** since the toDerivative method requires two sets of historical time series. 
* Added **CouponIborFxReset**.
* Implemented **CouponIborFxResetDiscountingMethod** for PV, PV sensitivity and currency exposure. 
date should be called "iborIndexFixngDate" or something similar to avoid confusion with FX index fixing.