add an 'ensure' parameter to apt::ppa

...as sometimes we want to get rid of them. :)

We leave this a little loose; rather than simply requiring a boolean for
$ensure, we set the stage for doing an easy switch to also allowing 'purge' at
some point in the future.
Merge pull request #184 from RsrchBoy/topic/removable-ppas

add an 'ensure' parameter to apt::ppa