remove some redunancy in _BaseAxes

While working on #1461, I've found that the `__init__` of `_BaseAxes`
takes some redundant arguments. Of the 7 arguments it takes,
3 (`xscale, yscale, label`) can be removed without harm, because
they are automatically used in `update(kwargs)` (they have matching `set_`
methods). Another two have different names as setters: `frameon` == `frame_on`
and `axisbg` ~ `axis_bgcolor`. Note that the later are mispelled, as they
refer to axes and not axis. The following call is actually legal:

    plt.subplot(111, axisbg='green', axis_bgcolor='red')

I do not know what to do with the other two arguments. (`sharex` `sharey`)

I've tried to remove this redunancy, keeping the old defaults
intact, and without breaking the api. My opinion is that axisbg and frameon
should be reomved, but api changes and deprectations are above my level.

See also issue #1461