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.
When you are updating the docs, would you mind checking them with http://jhudson8.github.io/fancydocs/index.html#create to be sure they work correctly?

Just paste the readme file content in there, click "Give Me The Fancydocs" then "Preview My Project".  It will ask you for your github org name - in this case, it doesn't matter what you put.  Thanks.