Add CMakeLists to enable building

Almost all of it was lifted from the `gr-ais` project so it may not be exactly what you want. :8ball: With this file it now will successfully `make`, however fails on `make install`. 

I utilized the typical:

```
$ mkdir build
$ cd build
$ cmake ../
$ make
$ sudo make install
```
which then resulted in

```
CMake Error at grc/cmake_install.cmake:36 (FILE):
  file INSTALL cannot find "/home/dan/src/gr-scanner/grc/invert.xml".
```
[A full build log](https://gist.github.com/danthompson/44e2c0f68fe3bd900936)

Upon further review, it appears the files listed in the `grc/CMakeLists.txt` don't exist in the repo. Its looking for `invert.xml` and `scanner_edacs_pkt_rx.xml`. I realize this CMakeLists file, and the other missing files may be "missing" for a reason. If so, feel free to close and I'll move on.

Excited :heart: for this, as I have an EDACS locally that I would love to use this on. Keep up the good work! :thumbsup: