allow use outside of CommonJS

I'm working to integrate Pikaday into a React project that doesn't use CommonJS. Rather than re-invent the wheel, I figured to look for prior art, and this project best fit the bill, except that it assumes usage of commonJS. I've adapted the module to use a loader I've used elsewhere that works with AMD and vanilla browser JS as well.

The tests pass with some warnings akin to:
```
WARNING in ./~/React/lib/toArray.js
There is another module with an equal name when case is ignored.
This can lead to unexpected behavior when compiling on a filesystem with other case-semantic.
Rename module if multiple modules are expected or use equal casing if one module is expected.
```

I'm not familiar enough with Karma to pinpoint exactly where these are coming from, though I suspect it's the capitalized arguments to the build() function. If you can give me a pointer about those warnings I'd be happy to take a further stab at them, but in the meantime I have integration work to do.