Chrono sync2013

Hi Jeff,

Pull request for chrono-sync2013 branch in my fork of ndn-js.

In the repository, js/sync folder contains the sync related files (each corresponding with chrono-sync, digest-tree or protobuf implementation); and tests/node/ contains a test-chrono-chat for the library.

I've done preliminary tests using my test-chrono-chat; 
One problem with SHA-256 hash input (recomputeDigest() in digest-tree.js) still exists, which makes the test not fully interoperable with that of ndn-cpp.

I'll do more tests, examine the comments and resolve this issue later.

Thanks,
Zhehao