Header paths are hard-coded in `pavement.py`

The include paths for CUDA and Thrust are hard-coded in `pavement.py`.  There should be some discovery mechanism for finding the headers available on a user's system.  For now, it's probably easiest to search the distribution default install paths for Ubuntu, and to support overrides through environment variables _(and possibly a `.config/cythrust/cythrust.conf` file)_.
