Update build files to allow creation of a Homebrew formula.

Hello,

I've written a [Homebrew](http://brew.sh) package for openalpr.  However, in order for it to work, a few hardcoded paths needed to be replaced with CMake variables.  The differences between the        current build and this patched version are:

- By default, config files would end up in /usr/local/etc, instead of /etc.  This could be overridden by passing `-D CMAKE_INSTALL_SYSCONFDIR =/etc` to the cmake command.
- Addition of an optional flag for building and installing alprd: `-D WITH_DAEMON=<boolean>` .  If omitted, defaults to `true`.
- Setting the proper directory value of `runtime_data` in `openalpr.conf` based on `CMAKE_INSTALL_PREFIX`.
- Addition of a "make uninstall" target.

Let me know what you think.  

Thanks!

Alex