pkg/index: Index more music-related properties.

Add disc and mediaref (a hash of the audio portion of the
file).

Also relocate taglib code to
third_party/github.com/hjfreyer/taglib-go.

Change-Id: I58364f525b787484af894663125163095256d7c6
rename pkg/serverconfig to pkg/serverinit

To avoid awkwardness with future pkg/types/serverconfig

http://camlistore.org/issue/339

Change-Id: If8a00941cdc269d6bcfe0f76c596154d5969bf0d
Merge "pkg/index: Index more music-related properties."
Add some common file signatures to magic.

Change-Id: I428933fdb5bb987113ae1bbd1161eca4c477b426
Merge "rename pkg/serverconfig to pkg/serverinit"
devcam: let test command take arguments, to run subset of tests
make.go: mirror over all magic testdata files too