- fix bug #1 (Editor return original editor instead secured one in fluent API
+ add support for setStringSet() and getStringSet() method introduced in API 11 but in compatible way with API 8 (own implementation)
+ add support for apply() introduced in API 9 with workaround for API 8
- fix Android 2.2 (API 8 compatibility)
- revert <distributionManagement> to original state
Threading, code review related changes.

Extract save method, which is now thread-safe on every API level, using
apply or commit.
Add 'default' implementation to apply and commit methods, to be more
compatible with Android original methods.
Change string set serialization method from text join to object
serialization.
Add tprochazka to the contributors

Thank you for your contribution!
Merge pull request #4 from kovmarci86/pr/2

Pull request for "Fix for Issue #1 and some other enhancements" by tprochazka.