Added image importing to posterous migrator

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?