Added cond function to Userlib

Added the cond function to Userlib, also added the associated tests
and documentation.

snippets/userlib-tests.rls

@@ -55,6 +55,36 @@
              (adder (lambda (x y) (add x y))))
          (eq? (reduce adder list) (add 1 2 3)))))
 
+   ('cond evaluates the first branch that returns true'
+     (quote
+       (let ((switch-one false)
+             (switch-two false)
+	     (switch-three false))
+         (cond (q
+             ((false (toggle switch-one))
+              (true (toggle switch-two))
+	      (true (toggle switch-three)))))
+	 (and (not switch-one) switch-two (not switch-three)))))
+
+   ('cond doesnt do anything if all the branches are false'
+     (quote
+      (let ((switch-one false)
+            (switch-two false)
+	    (switch-three false))
+       (cond (q
+           ((false (toggle switch-one))
+	    (false (toggle switch-two))
+	    (false (toggle switch-three)))))
+       (and (not switch-one) (not switch-two) (not switch-three)))))
+
+   ('cond returns the result of the branch that is evaluated'
+	  (quote
+	   (let ((variable false))
+	     (set (q variable)
+		  (cond (q
+			 ((true true)))))
+	     variable)))
+
    ('contains? finds elem in list'
      (quote
        (contains? (1 2 3) 1)))