Stop using CURL for http connections

Hi,

I've modified the code to stop using curl in favor or Excon, a great library for long http connections. It would be great if you could merge my changes on master and release a new version of the gem with them.

In the process, I've also added the Gemfile to work with bundler and make it easy to develop.

If you don't have any further interest in continue maintaining this library, please let me know, I'll be gladly take care of it.

Cheers