message: Add benthecarman to keys.txt
diff: 617C90010B3BD370B0AC7D424BB42E31C79111B8 Akira Takizawa
  E944AE667CF960B1004BC32FCA662BE18B877A60 Andreas Schildbach
  152812300785C96444D3334D17565732E08E5E41 Andrew Chow
  +0AD83877C1F0CD1EE9BD660AD7CC770B81FD22A8 Ben Carman (benthecarman)
  912FD3228387123DC97E0E57D5566241A0295FA9 BtcDrak
  C519EBCF3B926298946783EFF6430754120EC2F4 Christian Decker (cdecker)
  F20F56EF6A067F70E8A5C99FFF95FAA971697405 centaur
  