Reporting test coverage

[jakirkham]

Would be nice if the reported test coverage included line numbers. Better yet it would be great if it used a service like Coveralls or Codecov to display the results. This would make it easier to figure out which lines were missed and address them somehow.

[jakirkham]

PR ( https://github.com/alimanfoo/zarr/pull/118 ) and enabling Coveralls on this repo should fix this.