Added preliminary support for node height and slope cost
Now running make rebuilds the lib/pathfinding-browser.js and copies it into
the 'visual' application (even though there it's named 'visual/lib/pathfinding-browser.bin.js').
visual: The instructions box is now hidden by default. Supports the
selection of 'tools' to choose the mouse interaction mode.
Cell height properly supported in algorithms and UI.

Now supported: different modes of mouse interaction, "painting hills" on
the board (setting cell height with falloff based on distance). The A*
algorithm implementation has been modified to consider slope in both
movement cost and future distance heuristic.
Added some items to .gitignore.
Switched to unminified state-machine.js

While debugging, it's pretty common to have an exception thrown during a
state transition caught by the state-machine.js library. Using the
minified version makes debugging more difficult; since performance is
not a top priority, I switched to the unminified version.
Added Node.clone; Node.height lower bound set to 0

The path-finding algorithm is always given a copy of the Grid object,
which is made in the Grid.clone method, which used to discard the height
of all the nodes (it was set to 0 in the copy).  To avoid this kind of
problems, a new Node.clone method was added, which is easier to update
to make a proper copy of the Node object, in case its definition
changes.

Also Grid.paintHall never sets a node's height to a negative value.
UI overhaul. Added brush radius controls.

The UI has been made more practical for our uses (fixed position panels,
scrolling enabled, other slight changes)