Make script more portable with usr/bin/env

http://www.cyberciti.biz/tips/finding-bash-perl-python-portably-using-env.html

This allows get_iplayer to "just work" with perlbrew installations

http://perlbrew.pl/

While remaining back-compat with any traditional / standard installation / setup.
Removed extraneous line of code
Minor README edits
Added system plugin location for Windows
Added optional version string

$version_text can be set by installers to override formatted $version
printed in copyright notice (get_iplayer -V).  $VERSION_TEXT added to
web per added for same purpose.
Web PVR: change shebang to /usr/bin/env