work on specification and dictionary documents

The majority of this PR is for work I did on the specification and the dictionary.  The spec document got the most work.  The wiki page was way behind the current state of libxdi.  The dictionary page on the wiki was much closer to the current state of libxdi.

For both documents, I made a markdown document (for easy viewing on GitHub) and put them in the `specification` folder.  I was unhappy with the various options for converting the markdown files into PDF, so I rolled my own.  The bulk of the conversion uses [Pandoc](http://johnmacfarlane.net/pandoc/) to generate latex.  I then use a sequence of sed commands (it seemed portable...) to tweak pandoc's output.  The calls to pandoc and sed are in the `maketex.sh` shell script.  I then wrote a latex style file to make the output all pretty.

So the important parts of this PR are the `.md` files, `xdi.sty`, `maketex.sh`, and `.tex` files to wrap around the output of the `maketex.sh`.  I also deposited `.pdf` files for easy access.

I don't consider either the spec or the dictionary to be finished, but they are a lot closer.  This PR seems like a good starting point for further modification.

I would like to delete the `latex` folder from this repo.  I would also like to delete the spec and dictionary pages from the wiki.  That is, I would like to maintain the `spec.md` and `dictionary.md` files *rather than* the wiki pages.  This PR includes changes to the contents of the `latex` folder.  I started work on the spec document there before moving it to `specification`.  Since I want to discard the 'latex` folder, I didn't bother removing those changes from this PR.

I also started (and converted) a file called `background.md` that is an effort to explain the purpose of XDI using real science and data examples.  The `.png` files in the `image` folder go with it,  I am not very committed to that document, but it seems useful.

This PR also includes a few tweaks to the perl wrapper.

This PR also includes stuff in the `filemagic` folder.  Along with tools for having the unix file command recognize XDI files, I made a Windows icon and a Windows registry file to associate the icon with `.xdi` files.

Final comment: This burst of XDI-related activity started two weeks ago on a day I intended to sit down and pound away at unit testing for feff85exafs.  As I gathered up data to use in those unit tests, I knocked over a sequence of XDI-related dominoes, which I am in the process of picking up.  Once I get to the point that I am content with Athena's handling of XDI, I will continue working on feff85exafs unit testing.