Support for non-blocking sending messages to kafka producer

[nemanjamikic]

Zipkin is not critical component to work. If the kafka producer channel buffer is full messages should be just logged instead of waiting for space in buffer. There should be option when creating kafkaReporter to specify if you want to be non-blocking and time for timeout. This allows current request not to wait for space in channel buffer if there is none.

Here are changes a97fc6c
and pull requests: #149

[nemanjamikic]

Can somebody take a look?
CC @abesto @adriancole @rogpeppe @skaslev