amqp use custom queue/exchange name

[shenbaise9527]

start zipkin server with this:

RABBIT_URI=amqp://guest:guest@localhost:35020/test RABBIT_QUEUE=zipkintest java -jar zipkin.jar

zipkin-go can't send span to zipkintest

[coveralls]

Coverage remained the same at 73.59% when pulling 085fbb9 on shenbaise9527:amqp into 705fed4 on openzipkin:master.

[coveralls]

Coverage remained the same at 73.59% when pulling 085fbb9 on shenbaise9527:amqp into 705fed4 on openzipkin:master.

[coveralls]

Coverage remained the same at 73.59% when pulling 085fbb9 on shenbaise9527:amqp into 705fed4 on openzipkin:master.

[jcchavezs]

Thanks @shenbaise9527 I wonder if you could cover this in a test too.