Document isMounted

Text from @petehunt
Clarify componentWillMount behavior
docs fix back link in Examples
docs add jsx->js tab to live editors
Normalize line endings
Update homepage for new JSX/JS editor
Properly clear live editor on JSX compile failure
Simplify live editor execution logic
[docs] Tweak frontpage first example and jsx-compiler example
Update propTypes documentation.
Update broken link in 'why react' article
Normalize internal links in 'why react' article
Fix typo
fix incorrect link
add hash link for lifecycle section of working with the browser page
remove references to react-page
Fix docs typo
Fix animation example code

key should never be index into an array or there are bugs. Especially in
transitions.

Fixes #853
fix grammar mistake
Add documentation about React.renderComponent

Recently learned that components passed into `React.renderComponent` may not be the ones actually mounted. Also learned that it returns the mounted component. Added some documentation describing this.