Implement box-shadow with Azure Blur

[Afoucaul]

This PR is an attempt to solve #13.

I merged and updated what @liZe did back in 2013 in #149.
I also wrote the property rule to support box-shadow.

While I understand the points raised in #13 and #149, I strongly believe that it's better to have this implementation, improvable as it may be, rather than nothing.
Furthermore, @liZe did some awesome work, even though it's been deemed not so efficient.
Now that it's been integrated, we can consider improving the implementation, but at least, we can generate basic shadows.

Here's an example of how shadows are rendered:

<style>
div {
    width: 100px;
    height: 100px;
    background-color: green;
    box-shadow: 10px 10px 5px 5px black;
}
</style>
<div></div>

Not that bad looking IMO, and can be improved with a little CSS tweaking.

[Afoucaul]

So... Based on @SimonSapin's suggestion in #13, and his work on providing Python bindings for AzureBlur, I implemented the box shadow drawing using cairo and azureblur. It looks pretty awesome, to be honest.

This is the new shadow, using the same code as above:

I still have to work on the inset shadows.
What do you guys think about it?

Edit: I cannot seem to get azureblur with pip, although it's on PyPI. Does it need to be updated there? Else there's no choice but to install it from the sources...

[liZe]

I'm closing this PR for now (see #13).