To align a window to left/right/top/bottom half of a screen with mouse you should go to 'settings editor' -> 'xfwm4' and uncheck 'wrap_windows' property. Now you can drag window beyond the edge of the screen to tile it. But what are keyboard shortcuts for such a tiling? XFCE 4.10 has that functionality, but I can't find keyboard shortcuts for it.
Xubuntu keyboard shortcuts
By default, XFCE/Xubuntu tiling is a mouse only affair.
However, you do have the ability to define your own keyboard shortcuts:
Using settings manager --> window manager you have the Tile Actions as shown.
In the picture I have defined two custom keyboard shortcuts CTRL+T to tile the current window to the top and CTRL+G to tile to the bottom.
To define your own keyboard shortcut - double click in the shortcut field next to the action.