Allow dynamic fanstatic resources

This allows to create fanstatic resources during runtime.

This is pretty ugly and wouldn't be needed if we could defer
initialization of the fanstatic resources until the adhocracy config is
available.
Allow to declare instance stylesheets

This allows to declare a list of instance keys for which alternative
stylesheets of the form `adhocracy_{key}.css` exist under
`static/stylesheets`.

If a page within such an instance that instance's css is loaded instead
of `adhocracy.css`.
Merge branch 'per-instance-css' into develop (fixes #475)
Update pep8 (1.4.5 -> 1.4.6) and pyflakes (0.7.2 -> 0.7.3)

Pep8 now honors the `# noqa` comment, which disables pep8 checks for a
certain line (see jcrocholl/pep8#180).

This is useful for SQLalchemy `== None` filter comparisons and used all
quite a bit in Adhocracy model methods, so this should reduce our pep8
error counter quite a bit.
refactor sidebar
fix sidebar sizing
rm unneccasery nesting in sidebar
fix settings sidebar
force box-style on norm sidebar
Merge branch 'xi-feature-refactor-sidebar' into develop (fixes #474)
[scss] button gradients and mixins
[scss] rm bootstrap buttons
[scss] themable follow button
[scss] themable register button
[scss] themable feedback button
[scss] refactor buttons
[scss] remove duplicate button definitions from _forms.scss
[scss] fix delegate button vertical align
[scss] rm unused images
[scss] refactor tutorial