Make doc for githubio

OK, this is good enough to test now for sure.

```bash
$ make doc
```

should do the right thing, and 
```bash
$ make upload
````

should get you to the point that you have a libMesh.github.io in your directory, and manually going in there and doing a 'git push' will update the live web page.

The make rule to do this step automatically is in there but commented out at the moment.

A couple things:
1. I'd like to build the header/footer in ./doc/html/*.html from a common source, as described in a separate thread.
2. the snippets in the annotated examples look kind of boring - maybe we could process those through enscript too?

@jwpeterson, @roystgnr - what do you think?