overhaul package (ashi009#17)

This CL updates this package completely:

- add additional files to .npmignore.
- add coveralls badge in README.
- fix lint errors in README.
- use const/let instead of vars.
- use up-to-date `TypedArray` and `Buffer` APIs.
- add curly brackets to single-line `if`/`for`.
- make the package always safe to require.
  Instead of throwing at require time, it now exports a `calculate()` that throws.
- remove `engine` requirement from `package.json`.
- replace `blanket`, `travis-conv` with `nyc` and `coveralls`.
- replace `package-lock.json` with `yarn.lock`.
- specify dev dep versions to avoid future CI failures.
- test against node 6, 8, 10 and 12 on Travis CI.
- bump to v2.0.0, as the minimal node version required is 6.

1.0.4

Update travis node versions setting.

- Use "4" and "5" to test against latest v4.x.y and v5.x.y releases.

1.0.1

Bump to 1.0 and update benchmark result.

0.1.5

test on node 0.10, 0.11, 0.12