Add support for .length on arrays

This pull requests adds support for templates like 

    There are {{items.length}} items in your todo list

for data structured like 

    { 
       "items":[
              "Get Milk",
              "Get Eggs", 
              "Get Beer"
        ]
    }

Current implementation sees length as an undefined property  and yields 

     There are items in your todo list

Handlebars JS returns 

    There are 3 items in your todo list