Migrate dropwizard-spdy from NPN protocol to ALPN

Resolve #794
NPN is a deprecated protocol and ALPN it's successor.
Jetty doesn't support NPN for Java 8, so it makes sense to make a switch.

Look for reference:
http://www.eclipse.org/jetty/documentation/9.2.3.v20140905/alpn-chapter.html
Document the need to provide alpn-boot library for SPDY

Also add a link to Jetty ALPN guide, so users can choose the the
correct library depending on their JVM version.
Merge pull request #820 from arteam/alpn

Migrate from NPN protocol to ALPN, closes #794