Add built in CORS support

Fixes #122 by adding CORS support to Flask-RESTful. @kelvinhammond and @versae provided great input on this.

Flask encourages the use of decorators to augment functionality, so we've added CORS support as an included method decorator located at `flask.ext.restful.cors.crossdomain`.

Example usage:
```python
from flask import Flask
from flask.ext import restful
from flask.ext.restful.utils import cors

app = Flask(__name__)
api = restful.Api(app)

class Foo(restful.Resource):

    @cors.crossdomain(origin='*')
    def get(self):
        return "data"

api.add_resource('/')
```

The response from a GET request to the `/` endpoint will contain the following headers:
```
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: HEAD, OPTIONS, GET
Access-Control-Max-Age: 21600
```

These headers allow CORS for GET requests to this endpoint.