Add dist, bundler tests

[katelynsills]

Description

This PR generates three different dist files with rollup - an ES6 module version, a CommonJS version, and a UMD version. These are listed under pkg.module, pkg.main, and pkg.browser respectively.

This PR adds integration tests that run automatically with CircleCI to test that SES's dist files are usable with the main JavaScript bundlers and tools. We test SES with Webpack, Browserify, Rollup, Parcel, and a mocked version of what unpkg would provide after publishing the latest version to npm. There is a README.md in /integration-test with more detailed information.

Tests

• npm test to make sure normal tests still pass
• npm run build to build the dist files
• check that the dist files are ignored by git and eslint
• npm pack to check that the dist files are included in the npm tarball
• test the pre_release_test_integration_node_11 job locally with the CircleCI command line tool: circleci local execute --job pre_release_test_integration_node_11
• test other CircleCI jobs in the same manner

Closes

closes #94 Closes #95 Closes #85 Closes #63

[warner]

after #89 lands, let's rebase this (which means maybe deleting the first two commits)

[katelynsills]

after #89 lands, let's rebase this (which means maybe deleting the first two commits)

Sounds good!

[warner]

This looks good, as long as the test all still pass after rebasing it on #89 (which should land first, of course)