[PLAT-6676] Documentation change
[PLAT-6676] Exposing the fixing date properly

Exposing the fixing date properly so that it can be passed from the Forward Rate Agreement security to definition
[PLAT-6676] Index lookup
[PLAT-6676] Modifying constructor argument doc per review
Merge pull request #140 from OpenGamma/topic/plat-6676

FRA to ForwardRateAgreement