Commercial DRM support on DASH-IF site

[tobbee]

Already supported for pre-encrypted content (see #221), but there is currently no content on the DASH-IF site.

Intention is to use a license from EzDRM and produce encrypted content on the DASH-IF site.

This could be done on the fly or pre-encoded. With on-the-fly, it should be possible
to do adjustable low-latency by encrypting each fragment on the fly. That is harder
with pre-encrypted content.