Followup to previous patch set

[dinkypumpkin]

This a followup to the patches I posted yesterday.  These patches:

1. Make the Web PVR a bit smarter about validating search params for
"Add Search to PVR".  Search = .* is OK as long as additional advanced
search criteria are also specified.  The web GUI still enforces a
match limit.

2. Makes clear in man page/help screen that if you really want to 
download everything available with the CLI (with or without additional
criteria), you can still use --search=.* to force it.  This won't
affect existing PVR searches that have used .* as a search term.

The thrust of all this to try to make it more difficult for users to
accidentally queue up a download of the entire iPlayer site, something
that could easily happen before.  In future, it might make sense to 
change the way --limit-matches works and plumb it into the Web PVR,
but that was more surgery than I could undertake at the moment.

These patches have been incorporated into the get_iplayer Git repo.
Instructions for updating are here:

https://github.com/dinkypumpkin/get_iplayer/wiki/instructions