* Arcball::interrupt, mkdoc_rst updates

- Add Arcball::interrupt so that users can close the
  Arcball without needing a mouse release.
- Arcball docs complete with mini-example
- Exceptions and occasional segfault (?!) using the
  pybind11/tools/clang, enabled user to install a
  functioning version for their system if needed
    - Not sure what the underlying problem is, this fix
      is easier.
- Better code-block handling, indented code now remains
  indented in the python docs
- Added a `danger` replacement (future commit will have one)

* expose GLShader::Buffer (mixed GPU pipelines), docs

- GLShader::Buffer becomes a public class definition,
  added const ref acquisition of an attribute buffer
  via GLShader::attribBuffer
- GLShader docs completed
- Python bindings for GLShader::attribBuffer as well
  as GLShader::Buffer class
- Minor updates to CMakeLists.txt and docs/mkdoc_rst.py
  to ensure throws -> Throw and OSX bundle docs remain
  in py_doc.h when building on non-OSX

* Arcball active rotation as quaternion, docs fixes