Reorganize packages, closes #79

- All packages go into either cmd or pkg directories
- Most packages renamed
- Server packages moved into pkg/server
- sanitize moved into webui, as that's the only place it's used
- filestore moved into pkg/storage/file
- Makefile updated, and PKG variable use fixed

cmd/client/mbox.go

@@ -7,7 +7,7 @@ import (
 	"os"
 
 	"github.com/google/subcommands"
-	"github.com/jhillyerd/inbucket/rest/client"
+	"github.com/jhillyerd/inbucket/pkg/rest/client"
 )
 
 type mboxCmd struct {