Update Reverse.cs

Hi!!

I did a few changes, using StringBuilder better than string +=, because string in C# are inmutable, and when you do:
string a = "a";
a += "b"; 
You are creating a new string with "ab", and so on, but iwth StringBuilder it doesn´t happend, using Append() method.

I have removed RecursionRev too, I think that for most part of cases with Rev will be enough, is O(n) and I think is enough unless you have to process a really huge text.

I hope you like
Merge pull request #1 from Oscarbralo/patch-1

Update Reverse.cs