eve-http: register with app-layer api

The HTTP module of Eve didn't register itself with the app-layer
for HTTP. This meant that if no other HTTP logger was active, the
HTTP logging in Eve wouldn't work.

This patch makes the HTTP Eve module register itself correctly.

Bug #1133. https://redmine.openinfosecfoundation.org/issues/1133

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/230
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/150