Sample programs and some additional path operators.

I did this mainly to get a grip on github and the tooling for .md, so there's a lot more commits than necessary.

The main point is: The samples directory contains examples/samples ported to julia from the http://www.cairographics.org/samples/ (two are missing, curved rectangle - which is not an issue - and gradient to which a lot of patter functions are missing).

While doing this, i needed some extensions in Cairo.jl and constants.jl. I found more missing, but this will come later.

You should be able to open Samples.md which contains some text and the images check-in into samples.
You should also be able to run all samples in sample to re-create the images (the creation date is rendered into the picture, so you should recognize the change when you overwrite).

Anything else, comment here, or mail me.