Add information about sponsorship levels

More information is still needed to complete the Sponsorship Prospectus,
but this is the initial write up of the sponsorship levels. Once this
page is complete, the "Apply to be a Sponsor" page can be turned on.

References #16
Add some additional sponsor information

In addition to providing a tiny bit of information to potential
sponsors, this change also adds a call to action to get them over to the
application page.

The sponsorship levels should really be driven by the `Level` model.
That will need a few additional fields added to it.

Also the email address may change to a specific person's.

References #16
Merge pull request #21 from PyGotham/sponsors

Sponsors