Fixed the ch atribute elements always being set to ' ', regardless of input.

[SirFireball3521]

border.ch can now be defined in style.

The following code:

var example = blessed.box({
   top: 'center',
   left: 'center',
   width: '50%',
   height: '50%',
   ch: '@'
});

Will now create a box full of @ symbols. This is useful for a background. Also, you can now define the character for a bg type border in border.style.