Search PS3 4oD JSON data for MP4 versions

Uses the program name, series and episode to search the ps3.channel4.com
site for streams.

This is a revamped version of the PS3 site search code submitted in #12.  I removed some duplicated code and trimmed it to a slightly more idiomatic form.  

FWIW, I'm giving this feature a +1.  If a HDS programme isn't also on the PS3 4oD site, it's unlikely to come up in the brute force search of adjacent asset IDs.  Moreover, some recent programmes such as Black Mirror s02e03 and Complicit required search ranges of 50 and 270 respectively, so a few programmes may even slip through the cracks of the range search.