[react jsx transform] Spread attribute -> Object.assign

Add support for spread attributes. Transforms into an Object.assign just
like jstransform does for spread properties in object literals.

Depends on https://github.com/facebook/esprima/pull/22
Preparing to move defaultProps resolution and type validation to the descriptor

This copies the propType and contextType validation to a wrapper around the
descriptor factory. By doing the validation early, we make it easier to track
down bugs. It also prepares for static type checking which should be done at the
usage site.

This validation is not yet active and is just logged using monitorCodeUse. This
will allow us to clean up callsites which would fail this new type of
validation.

I chose to copy the validation of abstracting it out since this is just an
intermediate step to avoid spamming consoles. This makes more a much cleaner
diff review/history. The original validation in the instance will be deleted as
soon as we can turn on the warnings.

Additionally, getDefaultProps are moved to become a static function which is
only executed once. It should be moved to statics but we don't have a
convenient way to merge mixins in statics right now. Deferring to ES6 classes.

This is still a breaking change since you can return an object or array from
getDefaultProps, which later gets mutated and now the shared instance is
mutated. Mutating an object that is passed into you from props is highly
discouraged and likely to lead to subtle bugs anyway. So I'm not too worried.

The defaultProps will later be resolved in the descriptor factory. This will
enable a perf optimizations where we don't create an unnecessary object
allocation when you use default props. It also means that ReactChildren.map
has access to resolved properties which gives them consistent behavior whether
or not the default prop is specified.
Add simple React.createDescriptor hook

This is the minimal runtime change we need to switch the JSX syntax to resolve
to React.createDescriptor(...)