Add decryption of files

After some serious hell, we have decryption! I had to do something rather radical and revert back some stuff because for some reason the code we had running on @masnes computer was fork-bombing on mine. After about an hour of that I decided to just do start again. I guess Arch > Ubuntu? Ha! I feel bad about it since you put in some good work on this, and I just ended up re-doing a lot of the stuff we did earlier and stealing the commit history :fearful:

Please leave some comments on here for questions/comments/concerns. I'm sure there are some issues to be found since I spent hours battling in anger until I found [this little cheeky pain the the ass](https://github.com/ianks/fuse-encrypted-filesystem/compare/new-path?expand=1#diff-7119f0f0e8b0cb200945a95dc3a25bb4R334). 

Now we just need to implement similar logic for `xmp_create()` and `xmp_write()`.