Multiple updates/fixes for ClearKey support

Fixed problems with the JWK ClearKey PSSHs.

Updated the player to support our latest changes in ClearKey
(new SystemID and base64-encoding in PSSH)

Created a generic method for parsing PSSHs and retrieving systemIDs

Re-worked the key system selection mechanism to more properly deal with
common encryption.  If a cenc ContentProtection element is found,
the player will look through all the PSSH boxes to determine which
DRMs are supported in the content.  Then it will compare each of those
with the isTypeSupported() return value to select a key system.