Rename to_plist methods on AbstractObject and ObjectDictionary

Since the to_plist methods don't actually create a plist you can write directly to a file, it should not be named as such. Fixes CocoaPods/cocoapods.github.com#11

**Note:** This breaks backwards compatibility with anyone using `to_plist` since the method has been removed all together. Not sure if there's a depreciation plan for this.