Add all_functions template to list all functions
Fix navigation
Merge pull request #118 from agouge/list-all-functions

List all functions

Following up on #103, here I add an entry in the **Getting Started** section
called **Functions** which lists all functions currently available in H2GIS. 

It updates itself automatically using the [Liquid templating
system](http://docs.shopify.com/themes/liquid-basics).