Added Makefile for installing libxia and xip

I added a Makefile at the root directory of this repository that invokes `make` in `libxia/` and `xip/`, and also knows how to install `xip` and the `libxia` shared library, following the instructions from the `howto` file. This Makefile makes installation of `xip` and `libxia` easier, and obviates the need for the `howto` file.