Added missing .Net methods to String

I added the following methods which were missing in String:
Equals(string, string)
Concat(object)
IndexOf(char, int, int)
IndexOf(string, int, int)
String.Trim (Char[])
String(char[])
String(char[], int, int)
TrimEnd(params char[])
TrimStart(params char[])
Contains(string)
Join(string, params string[])
Join(string, params object[])
Join(string, System.Collections.Generic.IEnumerable<string>)
Join(string, string[], int, int)
Join<T>(string, System.Collections.Generic.IEnumerable<T>)
Replace(char, char)
LastIndexOf(char, int, int)
LastIndexOf(string, int, int)
IndexOfAny(char[])