Update docstring of the 'render_bundle' route

Initialyl, the `render_bundle` function was used with assets, but has
been modified to use bundles. The docstring now reflects the
modification.
Blueprint returns 404 is the bundle if not found
Tests blueprint URLs
More tests for the 'FileAsset' class
FileAsset does not accept absolute filename
Add informations about the differences of Flask-Compressor
Minor fixes in the README file
Cache processed asset content

When debug mode is disabled (`app.debug == False`) and the asset content
have already been processed (processors are applied to the raw content),
do not re-compute the asset content.
Add .travis.yml file