Added jsdoc 3 grunt task

**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) Configured to add jsdoc GFM plugin. Github Flavored Markdown can
be used in doclets.
2) Configured to use template at 'doctemplates/openseadragon', currently the default jsdoc3 template
3) Configured to put output at 'build/docs3'