MRG: Reorganize Specs into Top-level Specs Directory

This PR reorganizes all of the specifications into a top level "specs" folder. This is the first step in migrating the specification documents over to the "source" branch. 

It would also be useful on this PR to propose a few conventions to follow moving forward.

- [x] images go in the `img` directory
- [x] spaces are not used in any file names, with a preference for ` - ` or ` _ `
- [x] specifications follow the naming convention of `nidm-specification-name.html`
- [x] preference to use all lowercase(?) 

After we agree on how to handle the above, we will need to:

- [x] update specifications with correct paths
- [x] create a new PR to move `specs` to the "source" branch (if needed)

NOTE: [tips for pelican on github pages](http://docs.getpelican.com/en/3.3.0/tips.html)