Project proposals

Following the discussion at https://groups.google.com/d/msg/numfocus/tb-aKqEedos/jxlnz_ZIV5gJ , I had a look at adding a project proposal form to the NumFOCUS website.

It seems the entire website is static, using pelican, so I'm not sure what UI you had in mind for the form template? Do you have some backend servers that could store/process the submitted form?

This pull request avoids using a server. It uses the client's browser to open the proposal form as an email to info@numfocus.org, with pre-filled email body. The form is linked from the bottom of the `Projects` page, plus there's a link from the `Contact` page.

I also fixed three typos I spotted while looking at the sources.