Fix GridFlow focus issue

The GridFlow widget create a Pile which contains Columns, one per
row of the grid.  Because it creates the Columns widgets empty,
the Columns widgets all have their focus_position set to the default
of 0.  The only time the GridFlow widget will update the focus of
the Columns widgets when constructing them is if the widget it is
adding is the focus widget of the GridFlow.  This means that if
a GridFlow ends up with a row whose first widget is not selectable
and the current GridFlow focus position is not in that row, then
the entire Columns widget for that row will be considered not
selectable (as its focus position will remain 0).

Correct this by ensuring that the first selectable widget gets the
focus when a GridFlow creats a Columns widget (or the actual
GridFlow focus widget if it is in the row).

A similar fix is not needed for the Pile focus because as long as
the GridFlow focus position is set, the Pile focus will be as well.

Fixes issue #61.
fix bright bg rendering on linux term
turn deprecated slice methods into wrappers

__delslice__() and __setslice__() are deprecated as of python v2.6 [1]
and urwid does not support python versions pre-v2.6 (anymore).

Making these methods wrappers of __delitem__() and __setitem__()
de-duplicates code (and doctest's)

[1] https://docs.python.org/2.6/reference/datamodel.html
add doctest examples of negative indices
make __delitem__ support negative indices
Merge pull request #75 from rwarren/bright_bg_for_linux_term

fix bright bg rendering on linux term
Merge pull request #85 from hkoof/negative-indices

Fix negative indices in MonitoredFocusList
Merge pull request #74 from jeblair/master

Fix GridFlow focus issue
changelog for 1.2.2
changelog for 1.3.0
drop -dev from 1.3.0
add link to curtsies project
build docs: make sure our gh-pages branch is up to date
generate reference docs for AsyncioEventLoop
AsyncioEventLoop deps on front page
docs for AsyncioEventLoop
adjust Asyncio dep docs
start 1.3.1-dev
fix for bg bug introduced by #75