Added jsdoc3 grunt task

Addresses #8 

**To use:**

1) Add grunt-jsdoc to your site-build node_modules (it's in package.json
for future fresh installs):
npm install grunt-jsdoc --save-dev
2) Build the docs using the 'doc3' grunt task:
grunt doc3

**Notes:**

1) doc-conf.json has the configuration ([Configuring JSDoc with conf.json](http://usejsdoc.org/about-configuring-jsdoc.html))
2) Configured to add jsdoc GFM plugin. Github Flavored Markdown can
be used in doclets.
3) Configured to use template at 'doctemplates/openseadragon', currently
the default jsdoc3 template
4) Configured to put output at 'build/docs3'

jsdoc 3 Documentation at: [@use JSDoc](http://usejsdoc.org/)