SearchResult improvements

Added new states that were returned by the API. Used
http://help.mandrill.com/entries/21738186-introduction-to-webhooks as
reference because the documentation at
https://mandrillapp.com/api/docs/messages.html#method=search didn't show
all possible states.
Merge branch 'master' of git://github.com/shawnmclean/Mandrill-dotnet.git
fixing merge mess :)
Merge pull request #18 from moacyr/master

SearchResult update