fill properties with a value of currentColor should be preserved

@FWeinb First of all thank you for your effort and contribution with this library...saving us tons of time :+1: 

We're using inline SVG here and it looks like the only way to allow for more than one color to be applied to the inline def by multiple `use xlink` elements in a unique way (e.g. I want some inner path, shape, etc., to have a different color for 2+ instances) is to take advantage of the `currentColor` CSS3 property. Thus, this PR checks if it's a fill and if it has currentColor, and opts to NOT remove those.

There's a nice codepen I found while researching this requirement that is likely more descriptive than my last paragraph:
http://codepen.io/jjenzz/pen/AxLKz

Feedback: I hope you don't mind me giving feedback, but I found the `test/expected/includedemo-demo.html` particularly cumbersome since I was adding a new svg element. I had to force it in by calling `grunt svgstore` directly to get the file corrected, then copy over to the expected. Seems like it might be worth pruning out this expectation or making it somehow less fragile.