This abstracts the creation of DrmInfo to parse the EXT-X-KEY tag
differently per KEYFORMAT.
Issue #279
Change-Id: I2e187dcfc01f7306183c9090c54de58e81872bc4
HLS parsing will be split into parsing the text of the manifest into
shaka internal objects and parsing those into a shakaExtern.Manifest.
This will make logic more clear and allow to test ManifestTextParser
independently of HlsParser.
Issue #279.
Change-Id: I44349966d3617e2d208185e3ad125ecea89732b4