IJson and IDecompressor have no virtual destructor

[matthew-nagy]

Despite being pure virtual classes, their destructors have not been explicitly marked as virtual. This can cause derrived classes to not have their destructors called correctly.
It also makes clang++ on linux throw me a warning a mile long, and adding these changes to my forked version fixed these issues. If you'd like, I can create a pull request.

[SSBMTonberry]

Thanks for your issue! 🙂
Feel free to create a pull request on this.