Files directory structure

The proposed directory structure for images, pdfs and other asset files is:

files/<post-identifier>/<files>

where <post-identifier> may include a directory for grouping similar posts and the name of the post.

Examples:

files/news/2013-10-20-short-title/pretty.jpg
files/news/2013-10-20-short-title/someinfo.pdf

files/projects/space-probe/probe.jpg
files/projects/space-probe/circuit-diagram.png

files/our-space/people.jpg
files/our-space/3dprinter.jpg

Based on discussion in issue #6