Enable documentation conversion from markdown to HTML

[joshmcrty]

I updated the Gulpfile.js docs task to use Metalsmith to build documentation in HTML format. I created a very simple template using Bootstrap, and I've got some more ideas on how to improve/enhance the navigation and organization of the documentation that I'll try to work on soon.

I did a couple of quick passes to check links, but I'm sure there are still some that may be broken or malformed after the markdown->html conversion. All documentation pages should be here and the build works without error by running the gulp docs task. Essentially the GitHub version of SPServices documentation should now be "all there."

Maintainers should be able to generate HTML documentation after pulling these changes and running npm install to get the new dependencies before running the gulp docs task.

A few pervasive changes that were needed:

• Cleaned up YAML and reduced it to just a title property for now
• Used relative links for all internal links. Root-relative links i.e. starting with /docs/... don't work with the HTML files if you open them from your local drive (not on a web server with a /docs directory, that is). If one of the goals is to package documentation in the .zip file, then it'll be important for users to simply double-click the docs index.html file on their desktop and have it "just work").
• URL encode MSDN links with parentheses (e.g. with (v=office.12).aspx in them); these were breaking the URL parsing from markdown. It's a simply change, but one that we'll need to remember for future links.