allow analyze method to accept a URL as a string,  check if valid URI

proposed resolution for https://github.com/PRX/yahoo_content_analysis/issues/3

this supports

     YahooContentAnalysis::Client.new.analyze('http://www.cnn.com/2011/11/11/world/europe/greece-main/index.html')

and

     YahooContentAnalysis::Client.new.analyze(URI('http://www.cnn.com/2011/11/11/world/europe/greece-main/index.html'))