Add daemon support

Hey,

I added a basic daemon to avoid the long loading time of alpr. This is not very mature yet but it seems to work, I'm mainly starting this PR to get a bit of input about this from you, in order to "do the right thing".

The idea is that you start it like: `./alpr -c us` without a filename and then you can type filenames, or options like `-j -n 5 somefile.jpg` and it returns the data. This shaves about 300ms of loading time and it's great if you receive a lot of images from a live stream because it analyzes images at maximum rate (250ms per image instead of ~500ms).

I started it as a separate `alpr-daemon` executable but then realised I could reuse code for parsing options. Eventually I'll probably make this a separate executable but I need to work on a little language for communication with ALPR.