Ensure all the images are of the same size before creating leveldb

It is only until in the demo_compute_image_mean.cpp that the data sizes are checked. If the images do not have the same sizes, the time spent on converting them into leveldb will be much longer because the original sizes are usually much larger than the size required by the deep network. To save the time and frustrations of the users who forget to preprocess the images before converting them, validation should be done as early as possible with regard to the principle of ["Fail Ealy](http://www.washingtonpost.com/blogs/innovations/post/the-new-fail-fail-fast-fail-early-and-fail-often/2012/05/30/gJQAKA891U_blog.html), [Fail Often"](http://www.virgin.com/entrepreneur/fail-early-fail-often).

A mincepie based script to resize and crop the images in parallel is provided for convenience.