checked minor typo in Black formula
replaced short rate models by AJD modelling approach (DPS2000)
Replaced short rate section by AJD modelling

AJD models incorporate a wide class of short rate models. Furthermore,
the AJD approach allows for straightforward pricing applications
(options, (European / Am ericanbonds,spreads,a
Merge pull request #2 from kermittfrog/master

AJD update