Various README improvements (#224)

Improved the README:
- Added a table of contents.
- New introduction example that shows comparing two benchmarks, instead of just one.
- Consolidated and moved around a few sections.
- Generally improved/made more concise some of the wording.
- Fixed one or two errors/typos.
- Fixed the test status badge.

Also threw this in:
- Separate `unit` vs `e2e` tests in `package.json`.

Fixes #221
Fixes #223