Added support for template to render cells

Hello,

this patch is very small, but very useful in imho :)

To explain it, a little example. The data source contains objects like :

    {
      _id: "xxxxxx",
      username: "name",
      roles: [ "admin", "writer", "reader" ] 
    }

The goal is to display user roles as a list of (bootstrap) labels, and be able to remove each role individually.

the table settings (note the "tmpl" key in "tags" field):

    {
      fields: [
        {
          key: 'name',
          label: 'Username'
        },
        {
          key: 'roles',
          label: 'Roles',
          tmpl: Template.myRolesTmpl
        }
    }

then the template definition (the template context is the row object, so you can have access to any fields) :

    <template name="myRolesTmpl">
      {{#each roles}}
        <span class="label label-{{roleStyle}}">{{this}}<span class="remove-role">&times;</span></span>
      {{/each}}
    </template>

And the JavaScript coming with the template, for helpers and events

    Template.myRolesTmpl.events({
      'click .remove-role': function(e) {
        ... code to remove a role (this) to a user ...
      }
    });
    
    Template.myRolesTmpl.roleStyle = function() {
       ... code to compute a style name according to role name (this) ...
    }

I attach a screenshot of a similar code.

![reactive-table-tmpl](https://cloud.githubusercontent.com/assets/229339/2642333/677c1b82-bf07-11e3-9a51-b6fae7f70a85.png)

In previous versions, i was doing this using the fn fonction and the old fashion call to Template.myTemplate(), it was returning a string represention of the template. But it's not possible with 0.8.0, because a template is not a string any more, and it must be render() and insert() into dom using UI api (if you only get the string representation with toHTML(), you don't get events and reactivity).

I hope you'll like it, the description of the pull request is very long compare to the patch itself :D

Anthony.