[Core] New request handler mechanism + GZIP compression

* New request handler mechanism added, bringing a better separation of concern in the DandelionFilter: new handlers are now in charge of injecting the requested assets (main feature), displaying the debugger, reloading the bundles
* Custom handlers can be added thanks to the new SPI: com.github.dandelion.core.web.handler.RequestHandler
* The bundle graph viewer is now accessible with the "ddl-debug" URL parameter (previously "showGraph")
* The bundle reloading is now performed with the "ddl-reload" URL parameter (previously "bundleReload")
* Bundle definitions (whether XML or JSON) are now checked just after they are parsed (consistency, required configuration)
* GZIP compression is now supported thanks to a new request handler
** A new toolGzipEnabled configuration option is available
** By default, the gzip compression is done on several text-based mime types: text/html, text/css, application/x-javascript, application/javascript, text/javascript, text/plain, text/xml, application/xhtml+xml, image/svg+xml
* New encoding option, which is used to encode any content processed by the DandelionFilter

WARNING:
 * The Servlet API is now 3.0 minimum

Closes #10
Closes #33