String.prototype.substrReplace

@Description: Mimics php substrReplace replacing part of the string with another string from an index to a length
@File: String/prototype/substrReplace.js
@Author: Paul Visco
@Version: 1.0 11/19/07
@Param: String replaceWith The string to replace with
@Param: Integer start The index to start at
@Param: Integer start The length to replace
@Return: String The string with the replacement
@Example:

var myString = 'hello world';
var answer = myString.substrReplace('girl', 0, 4);
answer = 'girlo world';