Add "webdetails" plugin to see transaction on your browser

Hi there, here is my small contribution. I'm new to bitcoins, and I really like electrum to manage my digital wallet. Since I was checking my transactions online, to get more "human readable" information, I've added this plugin to open the details on blockchain.info. Before writing this plugin, I was doing a lot of copy'n'paste of the transaction hash to the Web to get the details.

By default, the URL is set to `http://blockchain.info/tx/{}`, but it is easily customizable using the settings dialog.

I can set a different default value, if you know a better provider.