Added custom_data and where_extra support

First, eve is awesome ! I wish I discovered it earlier !

I wanted that an authenticated user can only retrieve resources he created, this mean, I need to add a user key on insert ( **custom_data** ), and override wheres query ( **where_extra** ) so a user can only retrieve its own resources.

I'm not sure about the name of these functions, but here how it works for me:

```python
# -*- coding: utf-8 -*-
from eve import Eve
from eve.auth import TokenAuth
from flask import request


class TokenAuth(TokenAuth):
    def check_auth(self, token, allowed_roles):
        """For the purpose of this example the implementation is as simple as
        possible. A 'real' token should probably contain a hash of the
        username/password combo, which sould then validated against the account
        data stored on the DB.
        """
        # use Eve's own db driver; no additional connections/resources are used
        accounts = app.data.driver.db['accounts']
        return accounts.find_one({'token': token})


def custom_data():
    return {"user": request.authorization.username}
where_extra = custom_data

if __name__ == '__main__':
    app = Eve(auth=TokenAuth, custom_data=custom_data, where_extra=where_extra)
    app.debug = True
    app.run(host='0.0.0.0', port=8000)
```

Let me know if you want me to write tests (if you consider merging these changes), or if you have any feedback.

Thanks !