Add invert_channels option to TransformData

Added invert_channels option to TransformData. That would invert the order of the channels in datum and mean to be able to use the models released in #1138 with current level_db/lmdb and image_mean stored in BGR format.
This is a simple fix, what do you think @ksimonyan @shelhamer?

Ex.
~~~
name: "VGG_CNN_S"
layers {
  name: "data"
  type: DATA
  top: "data"
  top: "label"
  data_param {
    source: "examples/imagenet/ilsvrc12_train_leveldb"
    batch_size: 128
  }
  transform_param {
    crop_size: 224
    mean_file: "data/ilsvrc12/imagenet_mean.binaryproto"
    mirror: true
    invert_channels: true
  }
}
~~~