Add notes on live stream implementations

Start a doc to track some popular HLS live streams and the manifest features they utilize. Fill out info for Unicorn Media streams.

+# Live HLS Research
+This document is a collection of notes on Live HLS implementations in the wild.
+
+## Akamai HD2
+
+## OnceLIVE
+
+### Variant Playlists
+Once variant playlists look like standard HLS variant playlists.
+
+### Media Playlists
+OnceLIVE uses "sliding window" manifests for live playback. The media
+playlists do not have an `EXT-X-ENDLIST` and don't declare a
+`EXT-X-PLAYLIST-TYPE`.  On first request, the stream media playlist
+returned four segment URLs with a starting media sequence of one,
+preceded by a `EXT-X-DISCONTINUITY` tag. As playback progressed, that
+number grew to 13 segment URLs, at which point it stabilized. That
+would equate to a steady-state 65 second window at 5 seconds per
+segment.
+
+OnceLive documentation is [available on the Unicorn Media
+website](http://www.unicornmedia.com/documents/2013/02/oncelive_implementationguide.pdf).
+
+Here's a script to quickly parse out segment URLs:
+
+```shell
+curl $ONCE_MEDIA_PLAYLIST | grep '^http'
+```
+
+An example media playlist might look something like this:
+```m3u8
+#EXTM3U
+#EXT-X-TARGETDURATION:5
+#EXT-X-MEDIA-SEQUENCE:3
+#EXTINF:5,3
+http://example.com/0/1/content.ts?visitguid=uuid&asseturl=http://once.example.com/asset.lrm&failoverurl=http://example.com/blank.jpg
+#EXTINF:5,4
+http://example.com/1/2/content.ts?visitguid=uuid&asseturl=http://once.example.com/asset.lrm&failoverurl=http://example.com/blank.jpg
+#EXTINF:5,5
+http://example.com/2/3/content.ts?visitguid=uuid&asseturl=http://once.example.com/asset.lrm&failoverurl=http://example.com/blank.jpg
+#EXTINF:5,6
+http://example.com/3/4/content.ts?visitguid=uuid&asseturl=http://once.example.com/asset.lrm&failoverurl=http://example.com/blank.jpg
+```
+
+## Zencoder Live