Use OASIS for building

Reason:
Switch to a more flexible build system that will allow for easier refactoring and modularity in the future.

Files Changed:
This commit adds an '_oasis' file, as well as associated generated files. 
It replaces the Makefile with a generated one.

Dependencies added:
Oasis (`opam install oasis`)

P.S. Sorry for doing a huge PR at once. This was the first thing I could think of contributing when I saw the project.