Add fill_diagonal_offset()

Related to issue #1688.

Add a new function theano.tensor.fill_diagonal_offset(a,val,offset) which is currently only work for matrix  as well as the corresponding unit test.

However, as the input 'offset' is an integer, grad of it is undefined. How can I write an test_grad of an undefined grad?( the test can not past now because theano.gradient.grad_undefined will raise an exception ) 
Also if the sole purpose of the function is to provide documentation, I think you can just add the docstring below or above `fill_diagonal_offset_` and it should show up in sphinx.  Just test it to be sure.