pkg/index: Index audio duration.

Add pkg/media with code to calculate MPEG audio duration.
Index it in a "durationms" property.

Change-Id: Ifb6251657cadc365ef3f5667a0512fde17575560
Merge "pkg/index: Index audio duration."
search: basic lexer to supported quoted tokens in search expressions

Change-Id: I6c8c37e3b27ab947623bbe7431352a14aa029839
Merge branch 'master' of https://camlistore.googlesource.com/camlistore