Remove unneeded dependencies from install instructions
Add installation instructions on Mac
Add table of contents to README
Merge pull request #371 from OP2/doc/mac_install

* doc/mac_install:
  Add table of contents to README
  Add installation instructions on Mac
  Remove unneeded dependencies from install instructions
Add a custom set implementation based on a vector

Based on the dolfin::Set implementation.
Use vecset instead of flat_set to build sparsity
Add methods reserve and capacity to vecset
Preallocate 13 entries per row when building sparsity
Preallocate set entries heuristically based on arity
No longer require boost
Remove flat_set Cython header file