Revert cb1396b

Resolves #75 and Nexus-Mods/NexusMods.Paths#12.

The library was originally GPLv3, as seen in d54ca74,
but I decided to switch from GPLv3 to MIT in cb1396b.

Funnily enough, this licensing situation was one I created myself.
I maintain GameFinder (MIT) in my free time and NexusMods.Paths (GPLv3)
as part of my job. GameFinder is used in NexusMods.App and NexusMods.Paths
is used in GameFinder. Some of the core concepts in NexusMods.Paths were
developed to be used specifically in GameFinder.

Anyways, I don't particularly have a strong attachment towards the MIT
license, I just don't like changing licenses after two years.