Add MQTT-SN Encoder

[rasendubi]

Closes #27.

[drets]

   11   1 warning         Use fewer imports
 Found:
   import Data.Word (Word16)
   import Data.Word (Word8)
 Why not:
   import Data.Word (Word16, Word8) (haskell-hlint)
   12   1 warning         Use fewer imports
 Found:
   import Data.Monoid ((<>))
   import Data.Monoid (mconcat)
 Why not:
   import Data.Monoid ((<>), mconcat) (haskell-hlint)
   16   1 warning         The import of `Data.Monoid' is redundant
   except perhaps to import instances from `Data.Monoid'
 To import instances alone, use: import Data.Monoid() (haskell-stack-ghc)

[drets]

Apart from minor comments - LGTM :+1: