Once initiated mixins support.

Hi jhudson8 -

I added some new functionality to your lib.
Now, this lib supports once initiated mixins.
How it works:

You register mixin:
```
React.mixins.addOnceInitiated('mixinName', mixin);
```
Then, when you try to add this mixin multiple times to your React component:
```
React.createClass({
  mixins: ['mixinName("foo")', 'mixinName("bar")']
  ...
})
```
it will be initiated once and pass all params to your mixin 
```
mixin("foo", "bar")
```
instead of initializing your mixin twice
```
mixin("foo")
mixin("bar")
```

Let me know if you have any questions.