Fixed rate bond with schedule and calc

Hi there, I committed the change proposed [here](http://lists.r-forge.r-project.org/pipermail/rquantlib-devel/2014-October/000019.html) which basically extends the FixedRateBond API to provide a more flexible definition of the instrument which matches closely the QuantLib interface. Unfortunately this change is not backward-compatible. Please let me know you thoughts.