Allowing @ in proxy passwords and usernames.

Well, I don't have much to say, except that, the change that I made, is to allow for @ in the passwords when someone is using a proxy (like me!). Someone may change it to be more readable.

Test script:

```python
import requests
proxy = {'http':'http://test:test@123@myproxy.com:80'} # imaginary proxy, I used a real one in my case
print requests.get('http://google.com', proxies=proxy)
```

Before:
```python
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    requests.get('http://google.com', proxies=proxy)
  File "C:\Python2.7 For Chintoo\lib\site-packages\requests-1.1.0-py2.7.egg\requests\api.py", line 55, in get
    return request('get', url, **kwargs)
  File "C:\Python2.7 For Chintoo\lib\site-packages\requests-1.1.0-py2.7.egg\requests\api.py", line 44, in request
    return session.request(method=method, url=url, **kwargs)
  File "C:\Python2.7 For Chintoo\lib\site-packages\requests-1.1.0-py2.7.egg\requests\sessions.py", line 279, in request
    resp = self.send(prep, stream=stream, timeout=timeout, verify=verify, cert=cert, proxies=proxies)
  File "C:\Python2.7 For Chintoo\lib\site-packages\requests-1.1.0-py2.7.egg\requests\sessions.py", line 374, in send
    r = adapter.send(request, **kwargs)
  File "C:\Python2.7 For Chintoo\lib\site-packages\requests-1.1.0-py2.7.egg\requests\adapters.py", line 209, in send
    raise ConnectionError(e)
ConnectionError: HTTPConnectionPool(host='123@netmon.iitb.ac.in', port=None): Max retries exceeded with url: http://google.com/ (Caused by <class 'socket.gaierror'>: [Errno 11003] getaddrinfo failed)
```
Now:
```python
<Response [407]>
```