add script to store keys

store-key.py is a simple python script that takes a KID and a key
(both specified as hex strings) and creates a key file on the local
filesystem.

This allows playback of a stream that uses that KID, as cencdec will
look for this file, given the KID in the stream.
use GHashTable to store keys

Replaced the single key with a GHashTable that provides an index
from KID to key. This allows cencdec to support streams that use
multiple KIDs, such as in the case of key rotation.
changed to GST_TRACE for decrypt-in-place