[Monolog] added NotFoundActivationStrategy from MonologBundle

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

The NotFoundActivationStrategy class from MonologBundle is totally independent of the framework and should be part of the bridge instead. That would allow people to use it easily with Silex for instance.

ping @Seldaek