Add support for scribe transport

Nobody has claimed this yet.

The intial telemeters provided are http and kafka. For consistency with [zipkin-finagle](https://github.com/openzipkin/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
```

Start in build.sbt by reproducing the added zipkin("scribe") dependency, then run ./sbt assembly to inspect the libthrift 0.5.0-7 and 0.9.3 duplicate-class errors. Done means scribe transport support can be added without the libthrift assembly conflict.

Written by the indexing model from the issue text.