Add wsgi support

I added some support for bare wsgi applications.
Usage should be clear but here is a working example anyway:

```python
#!/usr/bin/env python
import wsgiref
import mimerender
import json

mimerender = mimerender.WSGIMimeRender()

render_xml = lambda message: '<message>%s</message>'%message
render_json = lambda **args: json.dumps(args)
render_html = lambda message: '<html><body>%s</body></html>'%message
render_txt = lambda message: message

@mimerender(
    default = 'html',
    html = render_html,
    xml  = render_xml,
    json = render_json,
    txt  = render_txt
)
def hello_world(environ, start_response):
    start_response('200 OK', [
    #    ('Content-Type', 'text/html'),
    ])
    return {'message': 'Hello. World!'}


if __name__ == '__main__':
    from wsgiref.simple_server import make_server
    srv = make_server('localhost', 8080, hello_world)
    srv.serve_forever()
```