WIP: parse . in formulas

Adds basic support for . in formulas like '~ .' to the highlevel
interface, to indicate all otherwise unused variables.

It *does not* support embedding . in arbitrary python strings like
'~ np.log(.)'. I suppose that would be nice, in theory, but it would
require rebuilding Python expression, instead of just using patsy's
formula language.

Let me know what you think. It definitely needs more tests,
documentation and exploration of the various edge cases.