Fix TypeError in extraports_* functions

Sometimes there is no extraports tag. In this case a TypeError is raised, which is confusing. Either a proper exception (MissingExtraports or something) should be raised, or None returned. This patch does the latter.
Update host.py
Update host.py
Merge pull request #41 from orf/patch-1

Fix TypeError in extraports_* functions
Fixed NmapService.tunnel handling
Merge pull request #44 from loopmachine/master

Fixed NmapService.tunnel handling
fix bug #43
Merge branch 'bug43'
issue 42: added NmapHost.vendor
Merge branch 'issue42'
issue37: added NmapService.service_dict
Merge branch 'issue37'
Merge remote-tracking branch 'origin/master' into es
added kibana/es example and fixed cpe