default backend to lmdb for image conversion and mean computation

lmdb is 10-15% faster than leveldb although it takes ~1.1x the storage.
This is usually irrelevant in prefetching since both are fast enough,
but more important lmdb allows multiple, concurrent reads for training
and evaluation several models on the same data.
fix cifar10 paths so they can be run from caffe root
switch examples to lmdb (except for custom data loaders)
Merge pull request #1128 from shelhamer/default-db-lmdb

default to lmdb for database storage
Fixed param order of cv::Size in cv::resize
Merge pull request #1152 from sguada/fix_cv_size_order

Fix param order of cv::Size in cv::resize
Back-merge

  Fixed param order of cv::Size in cv::resize
  switch examples to lmdb (except for custom data loaders)
  fix cifar10 paths so they can be run from caffe root
  default backend to lmdb for image conversion and mean computation