Remove hashie dependency

When running this with ruby 2.1.0 and Hashie 3.3.1, I wasn't getting any results (actually just a few of the first level fields) and after going through the source code and doing some debugging I found out that the problem was with hashie when creating a Mash from MultiJSON's resultant Hash.

To reduce the number of moving parts and dependencies, I introduced an extension to OpenStruct, which is a standard library class, that recursively generates objects out of hashes (the original OpenStruct doesn't support nested hashes)

Apart from that, the .rspec file was very dated and none of this would run with Rspec 2 or 3. As we're already at Rspec 3.1.5, I guess it's safe to change this. However if you want, I can make this a separate pull request. If that is so, I'd want to convert all tests from the old "should" to the new "expect" if you wouldn't mind.

If there's anything that's not clear or concerns you might have, let me know. I bumped the version to 2.0.1 but if you want I can exclude those commits.