Merge pull request #1293 from hypothesis/debugtoolbar

Add and register the debugtoolbar package
Disable the debugtoolbar icon on successful request

This gets in the way of the sidebar. It is possible to move it to
another location using debugtoolbar.button_style but there is nowhere
useful to put it where it doesn't get in the way.

Instead it can simply be loaded manually by visiting:

http://localhost:5000/_debug_toolbar/
Add a debugging section to the README

This introduces the debugtoolbar.