Fix for Multi-byte messages and multiple buffer messages

* Calculate PUB and MPUB lengths correctly depending on whether we have
a Buffer or a String
* Support a list of Buffer messages.
Fix bad timing on backoff tests

* More explicit conversion in ReaderRdy of backoff interval from
BigNumber to Number.
* Adjusted the BackoffTimer max to produce initial backoff values that
were sensible for the test.
Updated README, bumped version

* Updated the README changelog
* Bumped version in package.json
Adding npm badge in README
NPM badge now with downloads
DRY byteLength
Merge pull request #11 from dudleycarr/writer-fix

Writer fix