apt-cache, directory search fixes

* `apt-cache showpkg` => `dpkg -s`. 

  * `apt-cache` can show a package even when it is not available (https://github.com/kmsquire/VideoIO.jl/pull/12#issuecomment-52018914)

* add `/usr/lib/x86_64-linux-gnu`, `/usr/lib/i386-linux-gnu` to the default lib search path 

  * see the [Ubuntu Multiarch](https://wiki.ubuntu.com/MultiarchSpec)

* allow user to override the lib search path with `installed_libpath`.  

  * E.g., ffmpeg packages for Ubuntu are provided in a PPA, and install under `/opt/ffmpeg/lib`.