More API doc improvements

This addresses the remaining suggestions listed in #2054. It also adds more improvements to the API docs, and fixes some issues.

The highlights are:
* Landing page for the docs
  ![landing-page](https://cloud.githubusercontent.com/assets/211514/2988977/90e92c74-dc5f-11e3-975a-b0c0a71d8ffb.png)
* Better description of `change` events
* Do not list the `render` event for `ol.Map`