Move heading anchors 50px up to avoid nav bar

Fixes #447.

We do this by moving the actual anchored element up in the page without moving the actual text. (Apple uses a similar trick in their framed docs.) Now this looks a bit sillier on smaller screens but it's better overall.
Fix unmounting components mounted into doc element

If we are to unmount a component mounted into a document element we should
unmount it from document.documentElement and not from document.firstChild which
is a doctype element in this specific case.
Merge pull request #423 from andreypopp/master

Re-rendering components into a document leaks components
Make 'disabled' MUST_USE_ATTRIBUTE for compatibility with CSS [disabled] selectors.

When a ReactDOMComponent is created with the property `disabled: true` subsequently setting the property to `disabled: false` the HTML attribute `disabled="true"` was being left in the DOM.
Added docs for React cookbook section Introduction and Inline Styles React recipe
Moved cookbook recipes into separate directory. Updated nav_docs to loop through cookbook yaml. Added cookbook directory to js/ to add live editing of code samples
add tip style to style entry
add tip format for comparison
new entry for ternary expression in jsx
new entry on self-closing tag
small typo and code tag is now js highlight

Was html before bc github screws up the js highlighting for jsx.
fix permalink, temporarily remove script field of Jekyll
entry on returning only one node from `render`
style prop value shorthand, children prop manip warning
add line-height to unitless css props
entry on controlled input with `value` null
entry on willReceiveProps not triggered on mounting
minor spelling change on the "controlled input null value" cookbook entry
Added backgroundImage example to "inline styles" cookbook entry
all typos