RSSReader has the utility loadFromString

Hi Alex,

I'm new in Java and Android development, and your library is really useful and works perfectly, also saves me a lot of work :) Thanks! I'm developing the official MagPi application and I needed a RSS reader with a special method to parse a string containing a RSS document, because I'm using Async-http library to get content from the web in an asynchronous and threadsafe way .. I'd like send you this pull request in case you're interested to merge so I will add this repo to dependencies instead of our patched version :) I also added some file to ignore in .gitignore..

Thanks!