Enabled other month cell text and attributes to be changed with block parameter.

Modified the helper to allow setting the cell text or the attributes for the previous or next month with a block.

The code below used to only affect the current month, not the other months' cells (with a class "otherMonth" be default):
```
calendar(:year => 2005, :month => 5) do |d|
    [d.mday, {:class => "myCustomClass"}]
end
```
After this change, also other months' cells can be customized with the block.