Make uwsgi.log() accept an object (like print)

Like print(), accept a string or an object and log its __str__ or
__repr__ method output.  When debugging I use it like print(), but it's
not... and forget to str() things.

The method definition was also changed from METH_VARARGS (it doesn't
actually accept more than one argument) to METH_O, which makes sure you
only get one argument.  When calling PyArg_ParseTuple with the "O" flag
(objects) you can actually get more than one argument, so METH_O does
the hard work for us.

See http://docs.python.org/3.3/c-api/structures.html#METH_O