Remove `instance.componentDidUpdate` overriding since this is no longer
possible on React 0.11.

Fixes #98
Convert source module format from ES6 to CommonJS

The es6 module transpiler currently doesn't have a way to exclude 3rd
party libs which meant we have had to keep copies of React lib files
converted to use es6 module imports in our repo, this is bad practice as
it means we are unnecessarily locking react bootstrap to a specific
react version. This change swaps the module format to CommonJS which
(for the moment) has more mature tooling support allowing this
functionality.
Copy over internal React functions to /utils folder, update references.

Removed all references to react/lib files, and move necessary functions
into their own files in a utils directory including: classSet,
cloneWithProps, createChainedFunction, EventListener, merge, and
TransitionEvents. If/when these become available through React we can
remove these files.
Added huge hack to support doing `cloneWithProps` on both React 0.10 and
React 0.11.
Merge pull request #152 from pieterv/es6-modules-to-commonjs

Convert source module format from ES6 to CommonJS
Merge pull request #151 from pieterv/react-0.11-compat

React master compatibility
OverlayMixin container cleanup

* Loosen check on `container` prop so that if an object has a `getDOMNode`
method we use it, as per #160.
* Provide a stricter propType check for `container`.
* Provide a default prop for `container` that passes the propType check
in all cases.
* TESTS!