Pwnlib util web wget

Usage is pretty simple.

```python
>>> url    = 'http://httpbin.org/robots.txt'
>>> with context.local(log_level='silent'): result = wget(url)
>>> result
'User-agent: *\nDisallow: /deny\n'
>>> with context.local(log_level='silent'): wget(url, True)
>>> result == file('robots.txt').read()
True
```