Update to reflect changes in caps format and cenclib API.

- Latest qtdemux patch does not now use nested GstCaps in setting its
src pad caps; this patch updates cencdec to use the new format.
- Cenc types in gst-plugins-base are now MiniObjects; this patch updates
cencdec in line with changes to the API.
Update to use new caps format & pssh events.
Store keys in GBytes rather than GstBuffer.
Remove key property and get/set functions.
Formatting and whitespace changes.
Acquire key from file whose path is derived from the key's ID.
replaced plugins-good with plugins-base

the cenc library has moved to gst-plugins-base
added filename to error message for failing to find key
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
added dependancy upon libxml2 library

libxml2 is used to parse the PSSH event from dashdemux.
add Marlin system ID used in MPD

Marlin uses a different system ID to identify itself in MPD files
than is uses in pssh boxes in MP4 fragments. This commit adds support
for the MPD variant to the acceptable caps.
parse PSSH event from MPD

The PSSH data inside a PSSH event has a different format depending upon
whether it comes from an MPD file or from an MP4 file. In the case of an
MPD file, it is an xml string. In the case of an MP4 file, it is the
binary contents of a pssh box.

This commit adds support for parsing the xml data that is inside a
PSSH event that has been generated by dashdemux.
renamed PSSH to PSSI

Updated to match the changes to gst-plugins-base where PSSH was
renamed to PSSI.
Used gst-indent to re-format code
Rename GST_PSSI_* to GST_CENC_PSSI_*

Renamed GST_PSSI_ORIGIN_INIT_SEGMENT to GST_CENC_PSSI_ORIGIN_MOOV etc
to match the changes to gstcenc in gst-plugins-base
updated patch references

There are now multiple patches that have to be applied. To avoid having
to keep updating the README, it now points to the bugzilla ticket.
Updated example

The example has been updated to reference a public DASH CENC test
stream.