Requests library: add support for file uploads.

This refactors file uploads in general to no longer using
mechanize for creating the multipart request body.
Implement browser.reload method, reloading the current page.
Add tests for core browser functionality, such as contents, cookies etc.
browser.contents / browser.json: raise BlankPage when no page opened.
Add browser.cookies support for requests library requests.
New browser.on method, a lazy variant of browser.open.
Use requests library sessions, so that cookies and headers persist.