Additional listbox functionality
Additional listbox functionality

* Add title_align as a new keyword argument to construct a ListBox.  It
  allows justifying a ListBox to the left or right in addition to
  centering it.  This can be useful for constructing columns out of
  ListBoxes.
* Implement borderless sides.  If a ListBox is created with lines or
  corners set to the empty string then do not create a border element
  for them. This allows adjoining list boxes to share a single border,
  or two ListBoxes to seamlessly merge into one another.

Fixes #211

urwid/urwid,tonycpsu/urwid,inducer/urwid,tonycpsu/urwid,rndusr/urwid,wardi/urwid,rndusr/urwid,urwid/urwid,wardi/urwid,inducer/urwid,tonycpsu/urwid,inducer/urwid,rndusr/urwid,wardi/urwid,urwid/urwid