#34: adding support for xz2

This is my initial attempt as adding support for xz2. I'm not sure about what I've done in the libs.rs file.

• I've set memlimit to max in mainline tor it seems to be MEMORY_LIMIT (16 * 1024 * 1024) ?
• I've made to encoding lzma or xz but in mainline tor it seems to be "x-tor-lzma" ?
• The new_lzma_decoder fn returns a Result so I've unwrapped. Should probably handle the Error result. New to Rust haven't got a good feel yet for how this is done.

I also need to add tests and so on but at this stage it would be good to get a review before I proceed.