Performance improvement in AssertionInfo.

By using char const * instead of std::string we avoid significant
copying per assertion.  In a simple loop with 10000000 CHECKS on
my system, this reduces the run time from 9.8s to 6s.

include/internal/catch_assertionresult.h

@@ -40,15 +40,17 @@ namespace Catch {
     struct AssertionInfo
     {
         AssertionInfo() {}
-        AssertionInfo(  std::string const& _macroName,
+        AssertionInfo(  char const * _macroName,
                         SourceLineInfo const& _lineInfo,
-                        std::string const& _capturedExpression,
-                        ResultDisposition::Flags _resultDisposition );
+                        char const * _capturedExpression,
+                        ResultDisposition::Flags _resultDisposition,
+                        char const * _secondArg = "");
 
-        std::string macroName;
+        char const * macroName;
         SourceLineInfo lineInfo;
-        std::string capturedExpression;
+        char const * capturedExpression;
         ResultDisposition::Flags resultDisposition;
+        char const * secondArg;
     };
 
     struct AssertionResultData