Added support for passing basic HTTP authentication in the url.

I know that in the past the Python Selenium bindings didn't support passing auth credentials in the URL, but I just updated my selenuim library and drivers and it works now. This doesn't work with splinter because splinter establishes the connection first in `request_handler.py`.

For example, this now works with the latest Selenium bindings.
```python
from selenium import webdriver
driver = webdriver.Firefox() # or webdriver.Chrome()
driver.get('http://user:password@example.com')
```
The eqivalent with Splinter doesn't work.
```python
from splinter import Browser
browser = Browser()
browser.visit('http://user:password@example.com')
```
This will raise a 401 `HttpResponseError`

I have added a simple change to `request_handler.py` to support passing the credentials in this way.