Add and register the debugtoolbar package

This now renders stack traces in the browser rather than the vauge and
unhelpful "Internal Server Error" message. This is a godsend when
trying to debug issues with SCSS files.
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/
Restore default view to 'Screen'

As a part of the fix for #1231 in commit
https://github.com/hypothesis/h/commit/f66448fc3b2d1e07cbb491db73367b4b69e5f9a6
the default view was change to 'Document' from 'Screen'
but the annotations were not loaded into the sidebar as supposed
for 'Document' view, because the applyView was called before
the annotations were loaded.

Since it'd take effort to correctly implement Document view update
when the store loads new annotations, it is feasible to return
to the original 'Screen' view, which works well, because
views are planned to be refined, maybe redesigned.

Fix #1276
Merge pull request #1295 from hypothesis/1276-load-annotations-to-sidebar

Restore default view to 'Screen'
Add a debugging section to the README

This introduces the debugtoolbar.
Remove debugtoolbar as a production dependency

We now install it as an extra requirement.
Merge pull request #1294 from hypothesis/debugtoolbar

Improve debug toolbar integration
Don't install dev dependencies in prod
Update CHANGES.txt
Add issue numbers to changelog
Remove unnecessary leading slash in asset base url
Change maintainer in setup.py to eng@hypothes.is
Tagging v0.1.0
Include dependencies for front-end unit tests

Using Karma, Angular Mocks, Chai and Sinon.
Add Karma test runner for frontend
Setup unit tests for the username directive

Also refactored the directive to remove the dependency on the
controller. The ng-model attribute is still required but only to set
the scope.
Removed unused asset output targets
Fix versioneer.versionfile_build
Bump versioneer to v0.11