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.
First stab at refactoring clients
More refactoring, need to still look at tests
Some test fixing
Reverted default for service name, added/updated tests
Using convenience method for URI creation
Rebased to pull in header changes