Added ability to use list of proxies for each scheme. Proxies are randomly selected.

Basic Usage:
```python
import requests
r = requests.Session()
p =  {'http':['proxy1.com:81', 'proxy2:82', 'proxy2:82'], 'https': 'proxy4.com'}
r.proxies = p

print r.get('http://ya.ru')
print r.get('http://google.com')

print requests.get('http://ya.ru', proxies=p)

print r.get('https://www.google.ru')
print requests.get('https://www.google.ru')
```