Add proxy capabilities

This is a continuation of @seankross's pull request #139.

I can't say for sure whether my changes have caused this to work, or whether I just got lucky with the test proxy I chose (I tried several). 

The thing that may have been causing a problem was that we had a variable named `response`. httr has a function named `response()`, so I'm wondering if there was a namespace conflict. Particularly because my original attempts were failing at `httr:::is.response()`, which checks that the return value from GET is of class `response`. `httr:::response()` is the constructor function that attaches the `response` class attribute to that return value. Oh R....

I won't feel comfortable until others can reproduce this. Got my proxy from http://www.hidemyass.com/proxy-list/.

```
> my_proxy <- proxy_settings("151.200.170.146", 80)
> install_from_swirl("R_Programming", proxy = my_proxy)
Course installed successfully!
> swirl()
```