Sphinx inline code css

Addresses #1215. This makes "plain formatted" inline code look like github's formatting (with a shaded box around it). It does not apply the box to inline code that is a hyperlink. It also avoids doing it for some titles.

I think I got all the styles right. Browse through the built docs to check: http://kbarbary.github.io/astropy

While I was at it, I changed the google webfonts protocol to https. See commit message for details on that.

Side Note: I tried changing the issue to a PR using 

    hub pull-request -i 1215 -b astropy:master -h kbarbary:sphinx-inline-code-css 

but got this error message:

    Error creating pull request: Unprocessable Entity (HTTP 422)
    Not allowed to change field "issue"