Fallback to riak-admin if nothing else works

If escript as well as the http status api is found and working the
script will default to that, if however they can't be found or used the
script will try and fall back to using the command line riak-admin
instead.
Merge pull request #21 from bipthelin/bt-use-riak-admin

Fallback to riak-admin if nothing else works
Allow the event host to be set on the command line

Useful when trying `riemann-health` on a box to which you don't
have access to change the hostname—like Heroku where `hostname`
returns a GUID that's meaningless to you.