Implemetation of IP address ranges for no_proxy environment variable

Hello,

During OpenStack implementation we found out that it is not possible to use IP ranges in no_proxy environment variable. We needed to exclude whole range of IP addresses (172.28.1.0/24), not only domain names or particular IP addresses.

Now it is possible to specify no_proxy like

```
no_proxy=127.0.0.1,192.168.0.0/24
```
and request to http://192.168.0.20:5000/ won't go through http proxy.

I added tests for no_proxy function as well.

Kamil