ReactTransitions: Don't animate undefined children
Transitions: Handle undefined input to mergeKeySet

Gracefully handle undefined input to mergeKeySet.
Don't mutate passed-in props

Depends on #575; fixes #570.  Now we'll be in trouble if someone tries to share objects between calls to getDefaultProps but that was already true of getInitialState and I haven't heard any complaints there.

This is the same number of allocations as before; we're just copying props in the other direction. (In any case, the copy happens only on mount and there are a couple dozen costlier things we're doing already at that time.)
docs `select` `value` to control chosen option
Merge pull request #581 from chenglou/docs-select-value

docs `select` `value` to control chosen option
Merge pull request #440 from petehunt/new-taglines

New marketing copy
Merge pull request #537 from hojberg/dont_animate_undefined_children

ReactTransitions: Don't animate undefined children
Merge pull request #576 from spicyj/immutable-props

Don't mutate passed-in props
Add TapEventPlugin to ReactWithAddons

add eventpluginhub

Revert "add eventpluginhub"

This reverts commit 25560b025efb19b7e41a526c22b9f234d92a6e99.

injectTapEventPlugin

docs

add responder

document it