Modular model definitions

A way to import a protobuf model into another one, by specifying a layer of type IMPORT. We went for a file system analogy, to allow referencing layers and blobs from different parts of the model:

* Imports are renamed to “name of the import layer/name”, e.g. “conv_pool1/relu”.
* Imported layers can reference layers and blobs from the importing network as a parent folder, e.g. “../data”, or using an absolute path: “/data”.
* By default names are relative references, so existing network definitions are fine. They will resolve all objects in the root folder.

Imports can be configured using ${variables}, that are applied during load using simple string replace.

We modified mnist/lenet_train_test.prototxt as an example, by exporting the Conv/Pool part as a module that is imported twice.

In lenet_train_test.prototxt:

<pre><code>
...
layers {
  name: "cp1"
  type: IMPORT
  import_param {
    net: "examples/mnist/lenet_conv_pool.prototxt"
    var { name: "bottom" value: "/data" }
    var { name: "num_output" value: "20" }
  }
}
layers {
  name: "cp2"
  type: IMPORT
  import_param {
    net: "examples/mnist/lenet_conv_pool.prototxt"
    var { name: "bottom" value: "../cp1/pool" }
    var { name: "num_output" value: "50" }
  }
}
…
</pre></code>

lenet_conv_pool.prototxt:

<pre><code>
layers {
  name: "conv"
  type: CONVOLUTION
  bottom: "${bottom}"
  top: "conv"
  blobs_lr: 1
  blobs_lr: 2
  convolution_param {
    num_output: ${num_output}
    kernel_size: 5
    stride: 1
    weight_filler {
      type: "xavier"
    }
    bias_filler {
      type: "constant"
    }
  }
}
layers {
  name: "pool"
  type: POOLING
  bottom: "conv"
  top: "pool"
  pooling_param {
    pool: MAX
    kernel_size: 2
    stride: 2
  }
}
</pre></code>
I made these configurable in 6d18371. These were split into other commits since they aren't relevant to the purpose of this PR.