Test that Node is a function before using instanceof on it
omitClose for all void elements

According to http://www.w3.org/TR/html5/syntax.html#void-elements the following elements should not specify end tags:

area, base, br, col, embed, hr, img, input, keygen, link, meta, param, source, track, wbr
[Docs] document optional param for renderComponentToString
Add polygon tag to transform

Fixes #1144.
Fix docstring typo for pure render mixin
Add Object.freeze to polyfill list
Add srcSet attribute

Chrome beta supports this now: http://blog.chromium.org/2014/02/chrome-34-responsive-images-and_9316.html.
Fix stale usage of emptyObject in cloneWithProps-test.

After we run `require('mock-modules').dumpCache()`, the object exported by
the `emptyObject` module will no longer be identical to previously
exported objects, so tests like `expect(component.refs).toBe(emptyObject)`
will fail.

Note that this behavior only manifests itself in tests, because of course
we do not call `dumpCache` in production code.

We could consider storing the `emptyObject` globally to thwart the effects
of `dumpCache`, but it's more idiomatic simply to re-`require` the latest
version of `emptyObject`.
Re-require `cloneWithProps` as well, for consistency.

And don't mock `emptyObject`, since that might actually replace it with a
mutable/non-frozen object.
Merge pull request #1215 from benjamn/fix-cloneWithProps-test

Fix stale usage of emptyObject in cloneWithProps-test.
LegacyImmutableObject module

This copies the old implementation of ImmutableObject into LegacyImmutableObject.
Add Immutable

As titled. First of 3 diffs to add Immutable, ImmutableMap, and ImmutableObject. No logic changes.
Allow falsy values in statics

We found that the component would break if we set any statics with the
value 0. It turns out @chenglou already ran into this with
d71736b3ed665d1ffcd0c34a55510ce266a30eb2 but the statics code was copied
earlier, and still has this falsy check. Made the same change, updated
the unittest.
Merge pull request #1192 from spicyj/srcset

Add srcSet attribute
Merge pull request #1211 from spicyj/freeze

Add Object.freeze to polyfill list
Merge pull request #1208 from passy/patch-3

Fix docstring typo for pure render mixin
[docs] Add more words to downloads page
Merge pull request #1222 from spicyj/npm-dl

[docs] Add more words to downloads page
Fix empty JSX expressions sometimes emitting erroneous commas
Merge pull request #1180 from evanc/master

Test that Node is a function before using instanceof on it