Introduced cmake for building on linux

Hi Arturo,

I finally got around to putting together a prototype of the build system that I mentioned a while ago. Please DO NOT merge this pull request because it specifically addresses linux and will break the assumptions of the other platforms. This is just a prototype so you can see  how it would work on linux; I can extend it to support the others if you think this is something that you want in Modus. Try it out and see if you like how it works.

The big change here is that I have moved the headers into what was called the
src/ directory and renamed src/ to modus/. I also added CMakeLists.txt files to
the top level directory and to modus/. To build the code on linux, go to the top
level directory,

$ mkdir build
$ cd build
$ cmake -DMAKE_INSTALL_PREFIX=install ..
$ make all install

This will create an out-of-source build directory and build the modus library
under build/root/lib. The install prefix defines an installation directory
underneath the build/ directory, which is a nice safe place to have it during
development. When you want to install modus so that the rest of your system can
use it, change the prefix to /usr/local.