read_html() - Modifable user-agent
Webservers are currently rejecting calls from the `read_html()` function because of  unrecognized headers. 

e.g.,

``` python
import pandas as pd
url = r'http://www.transfermarkt.co.uk/en/premier-league/gegentorminuten/wettbewerb_GB1.html'
df = pd.read_html(url)
```

return an  "ValueError: Invalid URL"

The same seems to be true for the default parameters for the urllib2.urlopen() function i.e. urllib2.urlopen(url)

However if the user-agent is modified, calls are successful. 
e.g. 

``` python
import urllib2
opener = urllib2.build_opener()
opener.addheaders = [('User-agent', 'Mozilla/5.0')]
url = r'http://www.transfermarkt.co.uk/en/premier-league/gegentorminuten/wettbewerb_GB1.html'
response = opener.open(url)
html = response.read()
```

Can we modify the code to include a browser_agent variable to be passed for the read_html function?

This case has been raised as per the advice at Stack-overflow - http://stackoverflow.com/questions/18939133/how-to-modify-pandass-read-html-user-agent
