Version 0.8.0

Main changes:

• Browser and StatefulBrowser can now be configured to raise a
  LinkNotFound exception when encountering a 404 Not Found error.
  This is activated by passing raise_on_404=True to the constructor.
  It is disabled by default for backward compatibility, but is highly
  recommanded.

• Browser now has a __del__ method that closes the current session
  when the object is deleted.

• A Link object can now be passed to follow_link.

• The user agent can now be customized. The default includes
  MechanicalSoup and its version.

• There is now a direct interface to the cookiejar in *Browser
  classes ((set|get)_cookiejar methods).

• This is the last MechanicalSoup version supporting Python 2.6 and 3.3.

Bug fixes:

• We used to crash on forms without action="..." fields.

• The choose_submit method has been fixed, and the btnName
  argument of StatefulBrowser.submit_selected is now a shortcut for
  using choose_submit.

• Arguments to open_relative were not properly forwarded.

Internal changes:

• The testsuite has been greatly improved. It now uses the pytest API
  (not only the pytest launcher) for more concise code.

• The coverage of the testsuite is now measured with codecov.io. The
  results can be viewed on:
  https://codecov.io/gh/hickford/MechanicalSoup

• We now have a requires.io badge to help us tracking issues with
  dependencies. The report can be viewed on:
  https://requires.io/github/hickford/MechanicalSoup/requirements/

• The version number now appears in a single place in the source code.