Add ability to group contribution letters by contact (& possibly recurring contribution)

This is building on the Send PDF Thank you build by Samuel of Koumbit. Basically the code contains the concept of grouping pdf letters by contact but it was commented out.

Per discussion with dgg I plan to take it to then next level by

1) re-enabling the form elements
2) merging the letters by contact (possibly also by recurring contribution)
3) offering another text field to define a separator - so

{contribution.date} would render '2 Jan 2013' for one contribution but for more it would render '2 Jan 2013, 3 Jan 2014' OR '2 Jan 2013</td><td>3 Jan 2014' should that be selected as the separator (this allows for a minimal level of template design around this)
4) assign the {$contributions} variable to the template so others can build a more complex template
5) probably assign a {$totalAmount} variable as well

Because aggregate tokens are conditional on aggregate options I'm keeping them out of scope for now - but they would be a nice addition - I'm not sure if you would need to use js to alter the options list. I feel it could get complex from a UI point of view - also what if someone has put them in the text & then unchecks the box.

I like the idea of having a recurring block in the template .e.g {begin}
{contribution.date}

blah

{contribution.amount} {end}

that gets iterated for each contribution - possibly in a separate message template - but once again - out of scope