Add handling for multiple messages in one dgram

Sometimes the nREPL sends several concatenated messages at once. Bencode
decoding only decodes the first complete dict from the buffer, and this
sometimes causes the client to hang, because it throws away the rest of
the buffer, and the status array is in that part. Therefore the client
never sees the 'done' status of the server and never calls
`messageReceiveDone`.

This change iterates over the buffer by checking how many bytes were
decoded and slicing that off the beginning.

The status array check is only made for the last message because the
nREPL protocol spec states that the done status is only sent when
processing the message is done, done, done.
Merge pull request #2 from ilkka/handle-multi-messages

Add handling for multiple messages in one dgram