Adding JSON parameter to get_list method.

I added a json parameter to the get_list method, which creates two new tags {list_json} and {row_json} available between the get_list tag pair.  

Each tag spits out a json representation of the list (or row) which can be used between script tags to quickly make data available to page scripts.

Example:

```
{exp:stash:get_list name="gallery_images" scope="site" context="site" json="yes"}

{if count == "1"}
<script>
var image_data = '{list_json}';
</script>
{/if}

{/exp:stash:get_list}
```