Download and include images in posterous migrator.

ACHTUNG!! This is being merged into a non-master branch.

In https://github.com/mojombo/jekyll/pull/509, @markhepburn suggested that we import images from Posterous, and supplied the code. Thought it was a cool idea.

I haven't tested this (hadn't heard of Posterous before a couple weeks ago) but I think it worked for Mark.

Here's his original message:

> I've added image importing to the posterous migrator; the other media types could be added similarly but I thought I'd push this out there first.  (I'm not actually expecting this to go in as-is -- I'm new to both ruby and jekyll for a start!  It also includes a few arbitrary decisions however)
> 
> It does the following:
> 
> * downloads the "full" image from the media section of each post;
> * saves it in a directory imgs/<postname>;
> * removes the [[posterous-content:...]] block at the end of the content, and replaces it with an ordered list of images, including the base_path
> 
> (I figured a list should be easy to turn into a gallery, etc if people wanted to add their own js)
> 
> Is this useful?