fix and tests for half assignments... whew.

test/test_half_assignments.coffee

@@ -0,0 +1,36 @@
+num: 10
+num: - 5
+
+ok num is 5
+
+num: -3
+
+ok num is -3
+
+num: +3
+
+ok num is 3
+
+num: * 10
+
+ok num is 30
+
+num: / 10
+
+ok num is 3
+
+
+val: false
+val: or 'value'
+
+ok val is 'value'
+
+val: and 'other'
+
+ok val is 'other'
+
+
+val: null
+val: ? 'value'
+
+ok val is 'value'