Encode TraceIds in hex when adding them to HTTP headers

The values should be hex-encoded as specified in
https://github.com/twitter/zipkin/blob/master/doc/collector-api.md#http.

Fixes #25
Merge pull request #26 from klette/use-hex-encoded-trace-ids-in-http-headers

Encode TraceIds in hex when adding them to HTTP headers
Remove unused imports.