Use absolute path when storing images

closes #1983

Images were uploaded to a relative location, based on the current directory. So if you start Ghost using `node ~/Ghost/index.js`, it would put all images at `~/content/images`.

This changes it to use a absolute directory, based on Ghost's root directory (see `server/config/paths.js`).

@simme Can you please verify that it solves the problem?

@ErisDS Because the bug pollutes the user's file system, you might consider pushing the fix as an update to 0.4.