Added extract_keys with labels tool

[sguada]

Just added a simple tool to extract the keys (with associated labels) from a leveldb/lmdb dataset.

Usage:
    extract_keys [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

Example:

./build/tools/extract_keys -max_iter 10 ~/Data/caffe-train-lmdb
I1031 15:59:14.982887 12299 extract_keys.cpp:53] Starting Iteration
I1031 15:59:14.983891 12299 extract_keys.cpp:71] Processed 10 items.
00000000_n03891251/n03891251_402.JPEG 703
00000001_n07697313/n07697313_4046.JPEG 933
00000002_n01560419/n01560419_3918.JPEG 16
00000003_n03933933/n03933933_59123.JPEG 718
00000004_n02097298/n02097298_12209.JPEG 199
00000005_n03649909/n03649909_16243.JPEG 621
00000006_n02504013/n02504013_2796.JPEG 385
00000007_n03355925/n03355925_15226.JPEG 557
00000008_n01833805/n01833805_2266.JPEG 94
00000009_n03538406/n03538406_4090.JPEG 603

[sguada]

Included in #1568