Move content store implementation

Move the filesystem content store implementation to a
subpackage of content.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

content/fs/store_unix.go

@@ -0,0 +1,17 @@
+// +build darwin freebsd
+
+package fs
+
+import (
+	"os"
+	"syscall"
+	"time"
+)
+
+func getStartTime(fi os.FileInfo) time.Time {
+	if st, ok := fi.Sys().(*syscall.Stat_t); ok {
+		return time.Unix(int64(st.Ctimespec.Sec), int64(st.Ctimespec.Nsec))
+	}
+
+	return fi.ModTime()
+}