[Feature] Issue 245 - Variable Image Width

@mattyza, What do you think?

This pull request solves Issue #245

**What This Does**
I've added a new variable to include in FlexSlider called variableImageWidth. 

It causes the slideshow to first size all slides to the viewport width (as per normal). It then determines the minimum image height of the slides (assumption: one image/video per slide), 

**Required Slider Format:**
* Horizontal
* Slide Animation
* Not A Carousel
* Non-Image/Video Slides (assumption that the width of these slides is slider.computedW)

The codes checks for these requirements before enabling this feature `variableImageWidth = slider.vars.variableImageWidth && !vertical && !fade && !carousel`

Touch support, animation looping, and variable offsets are a part of this pull request. Meaning it all still works when this feature is enabled. 

**Note:**

I haven't minified the JS, but I can add that into the pull request if you like it. I can also make changes to the README.md and add a little bit of CSS if you like.

**Example Slide Image CSS For Variable Width Images**
```css
.flexslider .slides img {
    width: auto !important;
    height: inherit;
    max-width: 100%;
    max-height: 100%;
}
```

**Tested so far with:**
* jQuery v1.7.2
* iPhone 5 (Safari/Chrome)
* Firefox 24.0
* Chrome 30.0.1599.69
* IE 9