stream.hds: Support Akamai HD player verification, via “pvhash” parameter.

Reads <pv-2.0> element from manifest and sets up query string parameters to
be added to each fragment URL.
stream.hds: Calculate and cache player hashes via “pvswf” parameter.

Hashes are saved in a new “stream.json” cache file. The SWF file is requested
with an “If-Modified-Since” condition if a cached hash is already available.