incorporate ezxml into mx

[webern]

Closes #76

With this, the only remaining thing that is 'special' about ezxml is that it isn't in the mx namespace, and when we refer to it we

#include "ezxml/XFactory.h"

And not

#include "mx/private/ezxml/src/public/include/XFactory.h"

We retain its sub-directory structure and #include paths because it makes it easier to port the code back-and-forth with https://github.com/webern/ezxml

However, by removing its cmake file and no longer referring to it as add_subdirectory we eliminate any issues with installing mx.

In the future we should do what #78 says, and ensure that install package works during continuous integration runs.

[p-groarke]

I like this, much simpler!