Changes for the new MNIST fuel dataset.

[Thrandis]

Changes for MNIST refactoring. (mila-iqia/fuel#95)

[vdumoulin]

We'll need to change .travis.yml as well, I'm afraid. Here's what we'll need to do:

• Change lines 2-4 to

cache
  directories:
    - $TRAVIS_BUILD_DIR/data

• Remove lines 22-33 (those that download the raw MNIST files)
• Change line 34 to export FUEL_DATA_PATH=$TRAVIS_BUILD_DIR/data
• Right after line 40, add

  - |
      if [ ! -f $FUEL_DATA_PATH/mnist.hdf5 ]; then
        cd $FUEL_DATA_PATH
        fuel-download mnist
        fuel-convert mnist
        fuel-download --clear mnist
        cd -
      fi

[vdumoulin]

There's an even simpler way to make sure inputs are flat: pass flatten=('features',) to the MNIST constructor.

[vdumoulin]

To fix the failing doctest in docs/index.rst, just change line 89 to

>>> mnist_train = MNIST("train", flatten=('features',))

[Thrandis]

mhh some more doc to fix!