Update docgen artifact id to vertx-docgen and version to 3.0.0-SNAPSHOT
Added nexus snapshot repository to get the parent pom snapshot

Signed-off-by: Julien Viet <julien@julienviet.com>
Updated build readme with more accurate infos

Signed-off-by: Julien Viet <julien@julienviet.com>
Change default doc extension to adoc
HttpClientImpl.request() was losing the query params if passing a string absoluteURI

Signed-off-by: Adrian Gonzalez <adrian.gonzalez@ef.com>