Allow keyword arguments to etuple

[brandonwillard]

It would be nice to have the ability to specify keyword arguments to etuples. Specifically, this would make it much easier to set names for meta graph objects.

For example,

>>> from symbolic_pymc.theano.meta import mt
>>> from symbolic_pymc.unify import etuple
>>> etuple(mt.add, 1, 1, name='blah')
ExpressionTuple(( TheanoMetaElemwise(Elemwise{add,no_inplace}), 1, 1, name='blah'))