Add NSMutableString class

[programmingkidx]

Add the NSMutableString class.

This example program can be used to test this class:

package main

import "github.com/progrium/macdriver/core"
import "fmt"
import "reflect"

func main() {
	str1 := core.NSMutableString_Alloc().InitWithCapacity(100)
	str1.AppendString("Hello Go")
	str1.AppendFormat(" %s", "universe!")
	str1.AppendFormat(" %s: %10d %5d %.2f", "numbers", 98765432, 90, 91.9345545)
	fmt.Println("Value:", str1)
	str1.SetString("One")
	str1.InsertStringAtIndex(" Two <- InsertStringAtIndex()", 3)
	fmt.Println("Value:", str1)
	fmt.Println("Type:", reflect.TypeOf(str1))
}