support event handling for dynamic router

| Q             | A
| ------------- | ---
| Bug fix?      |  no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #131, #119
| License       | MIT
| Doc PR        | TODO @trsteel88 promised for later this week

this depends on https://github.com/symfony-cmf/Routing/pull/76