Fixes sprite magic import

[tkhyn]

Hi there,

While discovering and playing with pyScss to see if I could implement / plug in a little project of mine *, I noticed that the @import rule used with png files was completely broken, with remnants from old code (such as rule.context) that showed that the _at_magic_import had been forgotten for a little while.

I attempted to fix all that to get the correct compass behavior, and added the missing sprite_does_not_have_parent and sprite_has_selector functions referenced in the compass library for 'magic sprites' (http://compass-style.org/help/tutorials/spriting/magic-selectors/, this uses https://github.com/Compass/compass/blob/stable/frameworks/compass/stylesheets/compass/utilities/sprites/_base.scss).

Before that, I also fixed some issues with the windows file separators, using os.sep wherever possible / applicable (i.e. not when the input was an URL). Some of them were only for test success, others were bugs.

Note: Before merging, please check that these commits do not break the fonts.scss test, this is the only one I did not run (would have had to build fontforge with python bindings on windows and that bored me).

* the idea is to integrate an automatic SVG spriting mechanism that would work the same way as PNG spriting except it would optimize the SVGs first as well as generate PNG fallback sprites for incompatible browsers. I already have some python code to do that independently but now I want to be able to do @import "icons/*.svg"; in my SCSS.

[Kronuz]

Check the fonts.py. I'm using fontforge for building fonts from svg files (much in the way sprite maps work, but generating font files). Also, for svg, I've usted pycairo which has worker pretty well for me.

[tkhyn]

Ok. Will have a look at how fonts are generated to pick some ideas, thanks.
I indeed use cairo and rsvg for svg to png conversion (and scour for svg optimisation).

[tkhyn]

Feel free to let me know if there is anything that needs to be done on my side - or that you don't like, or that is not in the project's philosophy, or whatever - about this sprite import fix. Merging it with the current master branch is straightforward.

Btw, on the basis of this fix I have SVG spriting working in beta on my svg_sprite branch, using pyconizr which I'm developing in parallel. You may want to check it out, not much customization options yet (actually none, that'll come), but it does the job.