Fix rendering on iOS 7 devices

This fixes #5

The issue ended up being that UIFont's fontWithName:size: method
handles nil arguments differently on iOS 7. On iOS 6 passing nil
for the name meant the method would return nil, here it returns
Helvetica. As the size here is also nil, we ended up getting 0pt
Helvetica.

RPSyntaxHighlighter/RPSyntaxTheme.m

@@ -39,7 +39,8 @@ - (NSDictionary *)attributesForScope:(NSString *)scope
 
     UIColor *foregroundColor = [UIColor colorWithHexString:self.styles[scope][@"color"]];
     UIColor *backgroundColor = [UIColor colorWithHexString:self.styles[scope][@"background"]];
-    UIFont *font = [UIFont fontWithName:self.styles[scope][@"font"] size:[self.styles[scope][@"fontSize"] floatValue]];
+    NSString *fontName = self.styles[scope][@"font"];
+	CGFloat fontSize = [self.styles[scope][@"fontSize"] floatValue];
 
     if (foregroundColor) {
         attributes[NSForegroundColorAttributeName] = foregroundColor;
@@ -49,8 +50,8 @@ - (NSDictionary *)attributesForScope:(NSString *)scope
         attributes[NSBackgroundColorAttributeName] = backgroundColor;
     }
 
-    if (font) {
-        attributes[NSFontAttributeName] = font;
+    if (fontName) {
+        attributes[NSFontAttributeName] = [UIFont fontWithName:fontName size:fontSize];
     }
 
     return attributes;