Started to Implement Evaluation

Corrected Some Errors
Cleaned Code

MathPad/MPSumFunctionLayout.m

@@ -26,13 +26,7 @@
 - (CTLineRef)line
 {
     CTLineRef line = [self lineForPrivateCacheIndex:0 generator:^CTLineRef{
-        NSAttributedString *text =
-            [[NSAttributedString alloc] initWithString:@"∑"
-                                            attributes:@{NSFontAttributeName: self.font}];
-        CFAttributedStringRef attributedString = CFBridgingRetain(text);
-        CTLineRef line = CTLineCreateWithAttributedString(attributedString);
-        CFRelease(attributedString); // TODO: Is this release appropriate
-        return line;
+        return [self createLineForString:@"∑"];
     }];
     return line;
 }