add filter capability

Let me introduce myself.
I'm author of the [PEAR Net_Growl](http://pear.php.net/package/Net_Growl) package that allow to send notification to Growl from PHP on Mac/OSX and Windows.

More documentation about the package can be found at http://growl.laurent-laville.org/

As I wanted to propose/add a Growl handler to Monolog, but was a bit disappointed by lack of filtering logs capability in Monolog, I've choosen to propose you now this new feature as a PR.

I've of course modifed the source code, but also added unit tests and modified the documentation.
To read it more easily, you can have a look directly on my forked version at https://github.com/llaville/monolog/blob/FilterFeature/doc/usage.md#filtering-log-records

That will explains how to filter logs globally and locally, and allow a fine grain to accept/deny logs messages.

Laurent
@llaville If I create a handler with a specific log level I would not expect that a filter may change it. You should add a comment about this into the usage.md.