Add the jsmin processor

The jsmin processor is used to minify JavaScript code.
Add file extension to bundles

A bundle have now a file extension ('js' for JavaScript bundles for
example). The  extension is used to build URLs to bundles and assets.
This is needed when you use Frozen-Flask (or similar tools) to build a
static version of your website. This is also the first step to
statically build bundles and assets in an application deployment
scenario.
Add a hash in bundles and assets URL

URL for bundles and assets now contain a hash (md5 calculated from the
content). This helps browsers to cache ressources and force them to
fetch the new version if the content has changed.
Move exceptions in their own module

Exceptions are moved in their own module
`flask.ext.compressor.exceptions` to avoid circular import and simplify
imports in other modules.
Update README
Version 0.2.0