introduced height-control

added:

* `GRIDHEIGHT` variable
* `adjustheight` method
* `resizewindow_shorter` method

tweaked other methods so that they take height-var into account

refs #2
added explicit setters for width/height

Introduced functions:

* `grid.setheight(to)`
* `grid.setwidth(to)`

Reimplemented `grid.adjustwidth(by)` and `grid.adjustheight(by)` in terms of new functions (behaviour hasn't been changed)
Merge pull request #3 from indeyets/height

Height-related tweaks