added example and tests for exporting keras models

[kmader]

I added tests for simple CNN-based Keras models (https://github.com/riga/tfdeploy/issues/18) as well as the pretrained models which then cover the popular image classification models (ResNet, VGG, Inception), currently all of the tests pass but there is a hard coded list of UNSUPPORTED_LAYERS which are tolerated failures and show the corresponding missing operation on the tfdeploy side (https://github.com/riga/tfdeploy/issues/31)

Model 1: use_upsample could not be serialized unknown operation: ResizeNearestNeighbor
Model 5: use_lstm could not be serialized unknown operation: TensorArrayReadV3
Model 6: use_dropout could not be serialized unknown operation: Merge
Model 7: use_locallyconnected could not be serialized unknown operation: BatchMatMul
Model 8: use_repeatvec could not be serialized unknown operation: StridedSlice
Model 9: use_conv2dtrans could not be serialized unknown operation: Conv2DBackpropInput
Model 10: use_bn could not be serialized unknown operation: Merge