Add code coverage support via coveralls

https://coveralls.io/r/twitter/bijection

Signed-off-by: Chris Aniszczyk <zx@twitter.com>
might be better to do:

`./sbt ++2.11.4 clean coverage test`

or something for two reasons:

1) use the local launcher script so we control the sbt version.
2) use scala 2.11, which I think scoverage requires.