re-wrote README to reflect that PetIBM is also a toolbox
re-wrote README to reflect that PetIBM is also a toolbox

* add more information
* remove dependencies section; this section is duplicated since
  installation.md provides the same content
* add a list of MD files for quick start

mesnardo/PetIBM,mesnardo/PetIBM,barbagroup/PetIBM,piyueh/PetIBM,mesnardo/PetIBM