Adds tile handler to the perf server

perf/server/src/filetilestore/filestore.go

 package filetilestore
 
 import (
         "encoding/gob"
         "fmt"
         "os"
         "path"
         "path/filepath"
         "sort"
         "strconv"
@@ skipping 73 matching lines @@
                 return nil, err
         }
         f, err := os.Open(filename)
         if err != nil {
                 return nil, fmt.Errorf("Failed to open tile %s for reading: %s", filename, err)
         }
         defer f.Close()
         t := types.NewTile()
         dec := gob.NewDecoder(f)
         if err := dec.Decode(t); err != nil {
-                return nil, fmt.Errorf("Failed to dencode tile %s: %s", filename, err)
+                return nil, fmt.Errorf("Failed to decode tile %s: %s", filename, err)
         }
 
         return t, nil
 }
 
-// NewFileTileStore creates a new TileStore that is back by the file system,
+// NewFileTileStore creates a new TileStore that is backed by the file system,
 // where dir is the directory name and datasetName is the name of the dataset.
 func NewFileTileStore(dir, datasetName string) types.TileStore {
         return FileTileStore{
                 dir:         dir,
                 datasetName: datasetName,
         }
 }