Implement code coverage with nyc and coverall report

[heshan0131]

• npm run cover run test and report code coverage at coverage/index.html
• coverall.io integrate with travis CI to report code coverage

[ibgreen]

Very nice!

How does the coveralls.io integration work? Just installing the 'coveralls' module is enough?

Does coveralls.io provide a "badge" that we could put on the main README.md? @apercu

Maybe it is overkill, since it is difficult to add comments in this file, I am wondering if we should create a BUILD.md that contains some helpful comments about our build set up, in this case a couple of bullets about coverage.

Or we could add a section "comment": ["comment1", "comment2", ...}, and have a long string with comments. However, package.json is used included in the published module and even imported into the code for version checking, so we don't want it to be too long.

[balthazar]

Yes there is a badge :D

[heshan0131]

To implement coverall report, you just need to add --reporter=text-lcov to "cover": "nyc ..."

and yea coverall.io should automatically provide a "backge" for you.
There is not much extra effort needed, you have to add the repository to coverall.io, but that's just one click
https://coveralls.io/github/uber/deck.gl

[coveralls]

Changes Unknown when pulling 792a729 on test-coverage into ** on master**.