Add API information to documentation

Add a recipe to docs/Makefile. The recipe, "genapidocs", generates API
documentation directly from source code. Make "genapidocs" a dependency of every
other documentation generation command. Make the main documentaion page,
docs/index.rst, point to the API docs.

foo
Merge pull request #872 from Ichimonji10/apidocs

Add API information to documentation