Switch to Istanbul for code coverage

Istanbul is a code coverage tool that is much nicer to use and integrate in your workflow than jscov. It requires only its own NPM module and is compatible with ripple-lib without modifications.

I've also added integration with Coveralls (https://coveralls.io/r/ripple/ripple-lib) so we can track code coverage for each commit.

Also, it's time to re-enable Travis email notifications.

@wltsmrz / @sublimator - Please review / merge.