Set textarea value when changing content

At http://jsfiddle.net/spicyj/W4QLq/, typing into the textbox would cause
clicking the button to do nothing; now it should work.
Don't set value if it'll be a noop
Don't let textarea value change via textContent

Turns out my tests before weren't particularly useful because
receiveProps doesn't end up running componentDidUpdate since the
transaction never finishes. Now they use replaceProps instead (and I
verified that commenting out the "rootNode.value = ..." line makes the
tests fail, which wasn't true before).
Adds optional callback to `setState`

This commit adds an optional callback as a second argument to
`setState`, to be called after `setState` runs.

We never guarantee synchronous execution of `setState`, and as per
@phunt, we don't want to make that guarantee because we may eventually
batch calls to `setState`.  @jwalke agrees with him.
Updated docs and check for truthiness

Change api docs to reflect presence of the new argument.  In addition,
callback was change to require only a "truthy" value.
markdown syntax

Small problem with markdown syntax in syntax-highlighted block.
Add callbacks to all public-facing state/props methods

All public facing {set,replace,force}{props,state} methods now support
callbacks.
Merge remote-tracking branch 'origin/master' into textarea-update-value
Don't update value or textContent unnecessarily

_updateDOMChildren was already updating textContent so we don't need to
do it in _updateDOMProperties. Additionally, don't update .value if
it'll be a noop because it has side-effects (like moving the cursor) in
some browsers (like IE 9).

Refactor tests to be a bit more robust and a bit cleaner too.
Remove textContent tests; they break in phantomjs
Merge pull request #135 from vjeux/bugbuster

Add link roundup
Remove content property

Fixes #119.
Add support for empty XJS expressions
Fix Composition Level of Components w/o Owners

This fixes a bug with components constructed with no owners, for example:

  // Both the <div> and <span> have no owners.
  React.renderComponent(<div><span /></div>, node);

They should have a composition level of 1 and their keys should be prefixed with 0 to indicate they were created without owners. However, they currently incorrectly get a composition level of 0 (which means that //their// children will have keys prefixed with 0, which is wrong).
Throw on Missing Elements

This changes React to throw when `ReactID.getNode()` fails to find a node. This method is used by two call sites:

 - Implements `ReactComponent#getDOMNode`. This method already throws if a component is not mounted, and //all mounted components should be able to find their rendered root nodes//.
 - Used by `ReactDOMIDOperations`. These call sites aleady assume that `getNode` returns a non-null. Currently, if the node is not found, this is the site that fatals (and the stack trace is much harder to debug).

The error message should make it //a lot// easier to debug unexpected DOM trees. In particular, this will help track down all the places where the browser inserts `<tbody>` unexpectedly.
.reactRoot[base10] -> .r[base36]

Just a bit of byte savings for server rendering. Props to @benjamn for the base36 idea (and for making this diff easy).

With a little work we could probably get rid of the .r as well.
Merge pull request #136 from jeffmo/empty_expressions

Add support for empty XJS expressions
Merge branch 'textarea-update-value' of git://github.com/spicyj/react

Conflicts:
	src/core/ReactDefaultInjection.js
Delete files that accidentally got merged in

ಠ_ಠ
Clean up unused variables for lint