Add support for scribe transport

[klingerf]

The intial telemeters provided are http and kafka. For consistency with zipkin-finagle, we should consider adding scribe as well.

Adding a dependency on zipkin-finagle's scribe module, however, causes a version conflict with libthrift. To reproduce, first add the dependency:

diff --git a/build.sbt b/build.sbt
index acead10..264d03b 100644
--- a/build.sbt
+++ b/build.sbt
@@ -33,6 +33,7 @@ val `linkerd-zipkin` =
         twitterUtil("stats"),
         zipkin("http"),
         zipkin("kafka"),
+        zipkin("scribe"),
         scalatest() % "test"
       ),
       aggregate in assembly := false,

And then run ./sbt assembly. That produces a bunch of errors of the format:

[error] (*:assembly) deduplicate: different file contents found in the following:
[error] /Users/kl/.ivy2/cache/com.twitter/libthrift/jars/libthrift-0.5.0-7.jar:org/apache/thrift/EncodingUtils.class
[error] /Users/kl/.ivy2/cache/org.apache.thrift/libthrift/jars/libthrift-0.9.3.jar:org/apache/thrift/EncodingUtils.class