installation: Flask-Assets 0.9 and Jinja2 2.7.2

* Removing webassets from the requirements as its a direct dependency of
  Flask-Assets.

* Upgrading Jinja2 to 2.7.2.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
assets: working combined assets

* Using STATIC_MAP to distinguish between modules and external
  resources.

* Adds STATIC_URL_PREFIX for prefixing all the static files in
  production mode. Useful if you have a dedicated host for static files.

* Relying on ASSETS_DEBUG for webassets debug mode.

* Fixing the documentation for sphinx.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
base: documentation fixes

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
docs: invenio.ext.assets added

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>