overhaul package

[ashi009]

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.

[f5io]

Is the CI set to publish on PR? There's been a publish to npm 2 hours ago of 1.0.6. Its broken all of @google-cloud/storage.

[ashi009]

Is the CI set to publish on PR? There's been a publish to npm 2 hours ago of 1.0.6. Its broken all of @google-cloud/storage.

nop. Publishing is not in the CI.

Could you elaborate on what's broken? Given that v1.0.6 only changes the version of an optional dependency.

[f5io]

See #19

We might be an edge-case but if fast-crc32c@1.0.6 is used by hash-stream-validation@0.2.1 then @google-cloud/storage fails completely with the error in the issue.