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.
Introduced client module, abstracting commons away from jersey/apache impls
Merge pull request #27 from srapp/refactor-clients

Introduced client module, abstracting commons away from jersey/apache
Changed header encoding from toHexString() to toString()
Merge pull request #28 from srapp/negativeHex

Changed header encoding from toHexString() to toString()
RestEasy implementation now uses brave-client module
Merge pull request #29 from srapp/resteasy

RestEasy implementation now uses brave-client module
Added javadoc for brave-client classes.
Make classes package private that don't need to be public.
Make classes package private that don't need to be public.
Make classes package private that don't need to be public.
Add README.md for brave-client.
Update README.md
Update change log for 2.2.1-SNAPSHOT.
Update change log for 2.2.1-SNAPSHOT.
Update Contribution section.
Mention availability of brave-client.
[maven-release-plugin] prepare release brave-2.2.1