Syntax error messages: Use Oxford comma

In general PEG.js uses American English, where Oxford comma is
prevalent.

Part of #371.

lib/compiler/passes/generate-js.js

@@ -868,11 +868,18 @@ function generateJS(ast, options) {
       '      descriptions.length = j;',
       '    }',
       '',
-      '    return descriptions.length > 1',
-      '      ? descriptions.slice(0, -1).join(", ")',
-      '          + " or "',
-      '          + descriptions[descriptions.length - 1]',
-      '      : descriptions[0];',
+      '    switch (descriptions.length) {',
+      '      case 1:',
+      '        return descriptions[0];',
+      '',
+      '      case 2:',
+      '        return descriptions[0] + " or " + descriptions[1];',
+      '',
+      '      default:',
+      '        return descriptions.slice(0, -1).join(", ")',
+      '          + ", or "',
+      '          + descriptions[descriptions.length - 1];',
+      '    }',
       '  }',
       '',
       '  function describeFound(found) {',

spec/behavior/generated-parser-behavior.spec.js

@@ -1401,7 +1401,7 @@ describe("generated parser behavior", function() {
           var parser = peg.generate('start = "a" / "b" / "c"', options);
 
           expect(parser).toFailToParse("d", {
-            message: 'Expected "a", "b" or "c" but "d" found.'
+            message: 'Expected "a", "b", or "c" but "d" found.'
           });
         });
 
@@ -1433,7 +1433,7 @@ describe("generated parser behavior", function() {
           var parser = peg.generate('start = "c" / "b" / "a"', options);
 
           expect(parser).toFailToParse("d", {
-            message: 'Expected "a", "b" or "c" but "d" found.'
+            message: 'Expected "a", "b", or "c" but "d" found.'
           });
         });