Refactoring and python3 compatibility

Still not working. See issue #5 of upstream

https://github.com/evilsocket/google-play-downloader/issues/5
Use pycurl to download the file

Thanks to @daniellandau

https://github.com/evilsocket/google-play-downloader/issues/5#issuecomment-31620401

Important note: works only for python 2