Sample programs and some additional path operators (2nd try)

(After Tim Holy's comment (see #56), i did some experiments with rebase etc, but my best option was to redo the whole integration based on rollback and git push -f, so i need to learn still a few things about git/github).

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 pattern 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.