Some small corrections

Added example expression (hardcoded for development)
Added example change for expressions (hardcoded for development)

MathPad/MPDocument.m

@@ -58,4 +58,10 @@
     return YES;
 }
 
+#pragma mark Actions
+
+- (IBAction)changeExpression:(id)sender {
+    [self.resultExpressionView.expressionStorage insertString:@"abc" atIndex:6];
+}
+
 @end