Update CIFAR-10 example

I made some changes to `create_cifar10.sh`. I couldn't get it run with the `lmdb` option so I changed it to `leveldb`; it seemed that `convert_cifar_data.bin` cannot accept `lmdb` as an argument.

Moreover, `compute_image_mean.bin`'s output was:

    Computing image mean...
    F1101 06:29:56.293719  5732 compute_image_mean.cpp:67] Unknown db backend examples/cifar10/mean.binaryproto
    *** Check failure stack trace: ***
        @     0x7f36c1f08daa  (unknown)
        @     0x7f36c1f08ce4  (unknown)
        @     0x7f36c1f086e6  (unknown)
        @     0x7f36c1f0b687  (unknown)
        @           0x409e18  main
        @     0x7f36bea00ec5  (unknown)
        @           0x4090f9  (unknown)
        @              (nil)  (unknown)
    Aborted (core dumped)

I don't know whether this problem is only mine, but I made the changes, so they might be useful to someone else.

I also fixed the paths in `readme.md`.