Fix #447: Changed format for exporting models to MetaGraphDef for Tensorflow

[abhigyan7]

This PR makes Fabrik export models as MetaGraphDef instead of Graphdef, which fixes #447. Doing this allows models built in Fabrik to be imported into Tensorflow seamlessly.

[Ram81]

@haabhi nice work, can you test whether all models in FabrikZoo which were working before are still working after the change or not.

[yashdusing]

@haabhi are you going to make the changes in import_graphdef as well ?

[abhigyan7]

@yashdusing Yes, I am doing that at the moment. I am working on importing both metagraphdefs and graphdefs so the model zoo models and models exported from fabrik both work.

[abhigyan7]

@Ram81 @yashdusing
Now both binary and text metagraphdefs and graphdefs can be used to import tensorflow models in to fabrik. I have tested this with the prototxt models in the example/tensorflow directory and some random .meta and .pb files.