added functionality for electra classification head
Added functionality for an electra classification head following the google repo.  
Tested and trains fine with the run_glue script.  I was testing on my own repurposed classification problems so i didnt evaluate the actual GLUE accuracies, but can confirm that it is getting competitive results on my own problems.
Saw a few requests for this, thought it could benefit the community.

Originally I used a tanh activation in the classification head.  (which in my tests reliably performed better than a gelu activation)  But looking over the google code, it seems that they used gelu, so to keep the integrity of their paper im committing gelu here.

I also opted to make a ClassificationHead class instead of integrating a pooling layer in the base model class because i wasn't sure if that would affect other things, or run time in non sequence-classification tasks.
This is great, thanks @liuzzi!

We would need to add the corresponding tests. Can you do it, or would you like me to do it? If so, may I push directly on your fork?<|||||>@LysandreJik sure! i just invited you to collaborate.  I haven't been inside the transformers testing code so it'd probably be much faster for you to do.  Let me know if you have any questions/if i can help.<|||||>I just pushed the tests on it, thanks for your contribution!<|||||>Pushed a style change, rebased and merged into master. Thanks @liuzzi !