Added option to decide whether hash keys should be sorted

[haozhexu]

The hash keys are sorted which creates problems in use cases like simply editing and saving of existing plist, whose keys (in dictionaries) are not sorted but after saving the plist the whole file seems to me changed. This PR adds the option to not sort hash keys, by default it's true in order to be backward compatible with existing behaviour.

[patsplat]

Pardon the tardiness of my response.

If the output isn't sorted by key, would the hash end up being re-sorted arbitrarily because ruby hashes aren't sorted?

[patsplat]

Basically I'm not sure if this PR alone will do what you expect.

Note this library was originally developed to parse my iphoto library. It was being used on large plist files, with ruby 1.6.

I think I added the sort, way back when, because the output was always unpredictably different than expected. The sort in the output was arbitrary, but at least it made sense on some level.

If ruby hashes are now ordered, it could make sense to revisit this behavior. I'd prefer to ditch the sort entirely, though of course that may be a breaking change now.