Switch to mediaselector 5 API, added --mediaselector

Default for vpid data is now mediselector 5 used on iPlayer site:

http://open.live.bbc.co.uk/mediaselector/5/select/version/2.0/mediaset/pc/vpid/<vpid>

atk and asn parameters not used - works without them at present, though
may be necessary in future.

--mediaselector=4 will revert back to previous API