Add tasks to membership search - print labels & print pdf letter

Patch adds 2 tasks to membership search.

1 ) send a pdf letter (including basic membership tokens start_date, end_date, join_date, type, status, fee) from a membership search. The letters are 1 per membership
2) print labels. These can be per contact or per membership. If per membership is selected the merge-per-household & merge-per-address cannot be

Working 4.4 code is here https://github.com/eileenmcnaughton/civicrm-core/compare/labels?expand=1 - am currently getting it going against 4.2

Note that the tokens above come from an existing function but almost all the code in the commit is new functions. There are only about 4 lines that touch existing functions. I looked at generalising what was there but it seemed to be too risky to touch it too much so I tried to add the new functions in a way that was more generic than the existing functions. I think this provides a good basis for adding the same actions to other searches (preferably with more refactoring along the way)