Log to crossdock.T

[HelloGrayson]

We should add the ability to log to crossdock.T similar to testing.T. For the normal case, that would just log to stdout, but for the crossdock test, we can wrap the testing.T into a crossdock.T, and log to the test object. This will make the tests silent except in case of failure or when we use -v.

[yurishkuro]

@breerly it would also be nice to have a control var that would silence crossdock logging like below, even when running tests with -v. Maybe something similar to this: https://github.com/uber/jaeger-client-go/pull/37/files#diff-05d0732b45fb16a775bc3e5195a290aaR21

=== RUN   TestCrossdock
2016/09/01 19:52:59 HEAD http://127.0.0.1:54357/
2016/09/01 19:52:59 Client is ready, beginning test...
2016/09/01 19:52:59 HEAD http://127.0.0.1:54358/
2016/09/01 19:52:59 Client is ready, beginning test...
2016/09/01 19:52:59 GET http://127.0.0.1:54358/?behavior=trace&s1name=go&s2name=go&s2transport=http&s3name=go&s3transport=http&sampled=true