Changed game-repl prompt.

Customized the prompt of the repl. Now it looks a little bit nicer :-)
Added dot-name function

this function replaces non alphanumeric with underscores. Will be used in the
exercise to change lisp symbol identifiers like living-room into stuff that
graphviz can understand, i.e. living_room.
Added dot-label

This function, which is kind of weird, has the purpose of cropping long strings
into short strings. The idea is not to overload the graph with too much
information.
Added function to print labels in graphviz dot

Given an assoc list, which has a pair of (node . description), the  function
added will print in dot, this info
Added edges->dot to draw the dot graphic

With the previous nodes->dot we could define the states or nodes in the
graphic. With this function we connect them, and add a label to the transition.

[1]> (edges->dot *edges* )
LIVING_ROOM->GARDEN[label="(WEST DOOR)"];
LIVING_ROOM->ATTIC[label="(UPSTAIRS LADDER)"];
GARDEN->LIVING_ROOM[label="(EAST DOOR)"];
ATTIC->LIVING_ROOM[label="(DOWNSTAIRS LADDER)"];
((LIVING-ROOM (GARDEN WEST DOOR) (ATTIC UPSTAIRS LADDER)) (GARDEN (LIVING-ROOM EAST DOOR)) (ATTIC (LIVING-ROOM DOWNSTAIRS LADDER)))
[2]> (quit)
Bye.