Add Harmonize function and options

adds the ability to blend a common color over the colors in the scheme to create a 'harmonizing' effect. (a la [Erica Schoonmaker's technique](http://methodandcraft.com/videos/creating-harmonious-color-schemes)) 

Sample configuration:

```scss
// Do you want to “harmonize” your color scheme with photoshop-like blendmodes?
// See Erica Schoonmaker's technique: http://methodandcraft.com/videos/creating-harmonious-color-schemes
// Modes:
// null (no effect), normal, multiply, lighten, darken, darkercolor, lightercolor, lineardodge, linearburn,
// difference, screen, exclusion, overlay, softlight, hardlight, colordodge, colorburn, linearlight,
// vividlight, pinlight, hardmix, colorblend, dissolve, divide, hue, luminosity, saturation, subtract
$cs-harmonize-mode    : null;
// What is your common harmonizing color?
$cs-harmonize-color   : $cs-primary;
// How strong do you want the effect to be?
$cs-harmonize-amount  : 10%;
```

I also added a Bundler Gemfile and did some work on the test page in order to visualize this.