Re-org, cleanups, and add links to Recast and Esprima.

- Moved let-er into "Other", since it supports a feature that was not accepted into ES6
- Added links to Esprima and Recast in "Other" section
- Small cleanups and re-org in the "Transpilers" section
  * Added a link to es-arrow-function
  * Clarified that es6ify is based on Traceur