Added extract_keys with labels tool

Just added a simple tool to extract the keys (with associated labels) from a leveldb/lmdb dataset.
```
Usage:
    compute_image_mean [FLAGS] INPUT_DB [OUTPUT_FILE]

  Flags from tools/extract_keys.cpp:
    -backend (The backend for containing the images) type: string
      default: "lmdb"
    -max_iter (Max number of iterations (0 means extract all)) type: int32 default: 0
```