Pwnlib util web wget

Usage is pretty simple.

```python
>>> url    = 'http://httpbin.org/robots.txt'
>>> result = wget(url)
[*] Downloading 'http://httpbin.org/robots.txt'...
[*] Downloading 'http://httpbin.org/robots.txt': 1 / 30 B
[+] Downloading 'http://httpbin.org/robots.txt': Done
>>> result
'User-agent: *\nDisallow: /deny\n'
>>> wget(url, True)
[*] Downloading 'http://httpbin.org/robots.txt'...
[*] Downloading 'http://httpbin.org/robots.txt': 1 / 30 B
[+] Downloading 'http://httpbin.org/robots.txt': Saved data to 'robots.txt'
'User-agent: *\nDisallow: /deny\n'
>>> result == file('robots.txt').read()
True
```