Refactor pollers to allow better extensibility

* Introduce pollerCollections to allow the addition of custom pollers
* HTTP and HTTPS pollers now specialize a BaseHttpPoller, to reduce code duplication
* Catch more errors to avoid crashing the app when a poller is badly set
* Allow plugins to extend both the monitor and the webapp (warning: changes plugins signature)
* Add architecture schema in the README
* Make Dummy Target more verbose on the console
* Check can store pollerParams, and pings can store pollerDetails
* Monitor and dashboard emit new events
* Plugins can now add details to checks, abilities to pollers, and store additional details about pings
* Move Pattern Matcher logic to a plugin
* Update the README accordingly