[patch] Add template decorators

[rjwittams]

This patch includes two decorators for making template tags easier to create.

@simple_tag : 

This one is for converting functions that return a string into template tags. eg.

definition:

@simple_tag
def monkey_tag(verb):
    return "I %s no evil" % verb

use:

{% monkey_tag "hear"%}

arguments are resolved using template.resolve_variable -> ie currently limited to variable references and constant strings.

@inclusion_tag 

This decorator turns a function returning a dictionary into a template tag that renders another template inline using that dictionary as the context. The decorator takes three arguments :
path - required path to template
context_class - kwarg to override the default context created
takes_context - kwarg boolean. If true, the first argument of the function must be called 'context', and the context of the calling template will be passed into this argument. Useful if it is heavily context dependent and there would be a huge number of arguments.

The point of not just using the parent context is to make it easy to tell which context variables are used in which templates, ie avoid dynamic scoping hell.

eg:

@inclusion_tag('farming/cow_detail')
def cow_display(cow):
    return {
       'name': cow.name,
       'similar': find_similar_cows(cow)
    }

template : 'farming/cow_detail'

<div>
<h2>{{name}}</h2>
<b> similar cows: </b>
{%for cow in similar %}
   <p>{{cow.name}}</p>
{% end for %}
</div>

use:

{% cow_display cow %}

[rjwittams]

patch to add template decorators

[rjwittams]

I'm not sure about the naming/location of the file. Seems like we have a lot of things called template_<something>... is that telling us something?