Load HDF5 meshes from meshes folder by default
Consolidate argument parsing for aero, airfoil[_vector] demos
Register callback to automatically call op2.exit
Modularize aero demo
Modularize airfoil demo
Modularize airfoil vector demo
Add profiling option to aero demo
Add profiling option to airfoil demo
Add profiling option to airfoil vector demo
Add make target meshes to download example meshes
make meshes also generates triangle meshes
Merge pull request #178 from OP2/download-and-generate-meshes

Download and generate meshes

* Add a make target `meshes` to download and generate example meshes
  that don't exist yet.
* Add profiling option and some cleaning up of the aero, airfoil and
  airfoil vector demos.
* Register `op2.exit` to be automatically called when the interpreter
  exits.