Changed expr_uses_vars back to using a custom visitor. This is because we need to dive into bound variables from the linear solvers scope, so the visitor in ExprUsesVars is not sufficient.
Merge branch 'scoped_expr_uses_vars' into linear_solver
After merging in scoped_expr_uses_vars branch, changed has_vars method to use expr_uses_vars and deleted custom visitor.
Added EXPORT macro to linear solver functions. Grouped comments and made them readable by doxygen.
Merge master
Merge pull request #544 from halide/linear_solver

Changed expr_uses_vars back to using a custom visitor.
Merge branch 'master' of https://github.com/halide/Halide into no-shift-ocl-c