Adding support for subdivision stencils.

- Adding FarStencilTables and FarStencilTablesFactory classes
- Adding Osd EvalStencil context & controllers for CPU, OMP and TBB backends
- Adding the code example glStencilViewer
- Adding reST documentation
- Changing version to 2.2.0_dev
- Fix HbrMesh::Unrefine function
- Fix "CanEval" function in OsdVertexBufferDescriptor

Note 0: there is no stencil support for hierarchical edits

Note 1: there is no support for face-varying data stencils yet

Note 2: the current stencil factory is lazy but the caching system is not re-entrant