Add warnings to output for alpha levels in contrast check

[phillbaker]

This builds on #134 and the discussion there.

This is the initial PR, based on #134, to detect apha in either font text color or background color and issue a warning. It squashes all of the commits in #134 into one commit and adds a commit to focus the scope of the PR on just issuing warnings.

Three complications with calculating the "flattened" (non-alpha) representation of font color and background color discussed in #134 are:

• determining what is actually behind the text - elements may be positioned with css such that it's very hard to determine what element is the background of some or all of the text
• the css opacity element may introduce further aspects of transparency
• transparent background can reveal images where contrast cannot be calculated

It looks like a couple of other fixes were originally included in #134 that I left in that we can pull out into separate PRs if they're desired:

• ignore SVG objects when checking contrast
• Search for the smooth scrolling willChange: 'transform' background hack

This PR also adds an example to the test contrast html file.

[ironikart]

Thanks for this PR @phillbaker. I've made a few modifications and tested it out, seems to work well with the exception of some quirks with the unit test (I think phantomJS related because they work fine in the browser).

[phillbaker]

Thanks! Any chance of getting a release soon to include this?

[ironikart]

I'll put it on the radar, but it won't be this week (The current build process is quite time consuming and I need to allocate a decent slice to it).