Fix advection term in residual

Since the advection term is (\vec{u}\cdot\nabla) \vec{u}, grad should be replaced by nabla_grad in the weak formulation of the residual.
Added more mathematical details

Added text explaining why nabla_grad is used in this context. Also clarified the contraction of indices in the rest of the text by adding brackets where appropriate.
Add Eike and University of Bath to AUTHORS
Merge pull request #300 from firedrakeproject/fix_burgers_demo

* fix_burgers_demo:
  Add Eike and University of Bath to AUTHORS
  Added more mathematical details
  Fix advection term in residual