Add funcs for manipulating delimited lists in the dialplan

Adds PUSH and POP for appending to and retrieving/removing from the
end of a list and UNSHIFT and SHIFT for insert to and retrieiving/
removing from the beginning of a list.

Review: http://reviewboard.digium.com/r/230


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@190000 65c4cc65-6c06-0410-ace0-fbb531ad65f3