Encode Requests with UTF-8 to support German Umlauts in requests 

if i send have e.g an Recipient like this:

new MandrillRecipient("Andreas Lüdeke", "mail@bla.de")

mandrill returns a HTTP Status 500 wit message "you must specify a key value". This happens due to the fact that the ü is not properly encoded within the StringEntity. By Specifying the correct Encoding it works properly
Merge pull request #11 from aluedeke/patch-1

Encode Requests with UTF-8 to support German Umlauts in requests