Update readme.md files of cifar10 and mnist examples. Fixed broken links.

I made the following changes to examples/mnist/readme.md and examples/cifar10/readme.md files (for issue #1099):

1. examples/mnist/readme.md:
 1. Introduction: installation page link is broken. Changed the path to /installation.html
 2. Define the MNIST Network: lenet_train.prototxt doesn't exists. Changed it to lenet.prototxt (it can also be changed to lenet_train_test.prototxt, but this tutorial does not refer to lenet_train_test.prototxt file).
 3. Added a note about the lenet_train_test.prototxt file (for the definition of include: { phase:TRAIN } in lenet_train_test.prototxt file).
 4. Training and Testing the model: run 'train_mnist.sh', train_mnist.sh file doesn't exists. Changed it to train_lenet.sh.

2. examples/cifar10/readme.md:
 1. Introduction: installation page link is broken. Changed the relative path to /installation.html
 2. The model: cifar10_quick_solver_lr1.prototxt uses cifar10_quick_train_test.prototxt file, not the cifar10_quick_train.prototxt file.
 3. Training and Testing: added a reference to MNIST Tutorial on how to writte the network definition protobuf and solver protobuf files.