Emit transfer event during download

[charlietilt]

Summary

updated the client download method to emit a transfer event on an interval with the amount transfered thus far.

Example:

client = new Scp2.Client config
    client.on 'transfer', (buffer, bytes, totalBytes) ->
      console.log "Total Bytes transfered (#{localFile}): #{totalBytes}"
    client.download remoteFile, localFile, (error) ->
      callback error, localFile

[popomore]

Same as #61?

[charlietilt]

Agree, same features as in #61

[popomore]

And I'll close this.