Fix simpleDuration, modifiedDuration and convexity with Act/Act(ISMA)

http://sourceforge.net/p/quantlib/bugs/162/

This commit fixes simpleduration, duration and convexity with Act/Act(ISMA)

Tested with following code:

```
    Calendar calendar = TARGET();

    Integer fixingDays = 1;
    Natural settlementDays = 1;

    Date settlementDate(21, August, 2013);
    settlementDate = calendar.adjust(settlementDate);
    
    Date issueDate(15, August, 2013);
    Date interestAccrue(15, August, 2013);
    Date firstCouponDate(15, February, 2014);
    Date maturityDate(15, August, 2023);

    Rate coupon = 2.500;
    
    Period period = 6*QuantLib::Months;
    
    Compounding comp = Compounded;
    Frequency freq   = Semiannual;
    DayCounter dc = ActualActual(ActualActual::ISMA);

    FixedRateBond bond (settlementDays, 100.0,
                    Schedule(
                             interestAccrue,
                             maturityDate, 
                             period,
                             NullCalendar(),
                             Unadjusted,
                             Unadjusted,
                             DateGeneration::Forward,
                             false,
                             firstCouponDate),
                    std::vector<Rate>(1, coupon/100.0),
                    dc,
                    Unadjusted,
                    100.0,
                    issueDate,
                    calendar);
                    
    
    std::cout << std::setprecision(10);
    
    std::cout << "ISIN: US912828VS66" << std::endl;
    std::cout << "CUSIP: 912828VS6" << std::endl;
    std::cout << "Settlement date: " << settlementDate << std::endl;
    std::cout << "Issue Date: " << issueDate << std::endl;
    std::cout << "Interest Accrue: " << interestAccrue << std::endl;
    std::cout << "First Coupon: " << firstCouponDate << std::endl;
    std::cout << "Maturity: " << maturityDate << std::endl;
    std::cout << "coupon: " << coupon << std::endl;
    std::cout << "period: " << period << std::endl;

    Real price   = 102.25;
    Real accrued = bond.accruedAmount(settlementDate);
    Real npv = price + accrued;
    
    const Leg& leg = bond.cashflows();
    
    Rate yield = CashFlows::yield(leg, npv, dc, comp, freq, false, settlementDate);
    Time duration = CashFlows::duration(leg, yield, dc, comp, freq, Duration::Modified, false, settlementDate);
    Real convexity = CashFlows::convexity(leg, yield, dc, comp, freq, false, settlementDate);
    
    std::cout << "Price : " << price << std::endl;
    std::cout << "Accrued : " << accrued * 10000.0 << std::endl;
    std::cout << "NPV : " << npv << std::endl;
    std::cout << "Yield : " << yield * 100.0 << std::endl;
    std::cout << "Mod duration : " << duration << std::endl;
    std::cout << "Convexity : " << convexity / 100.0 << std::endl;

    std::cout << "PV 0.01 : " << npv * duration / 10000.0 << std::endl;

    std::cout << "CashFlow: " << std::endl;

    Integer n = leg.size();
    for (Integer i = 0; i < n; ++i)
    {
        std::cout << leg[i]->date() << " : " << leg[i]->amount() << std::endl;
    }
```

and verified to produce following:
ISIN: US912828VS66
CUSIP: 912828VS6
Settlement date: August 21st, 2013
Issue Date: August 15th, 2013
Interest Accrue: August 15th, 2013
First Coupon: February 15th, 2014
Maturity: August 15th, 2023
coupon: 2.5
period: 6M
Price : 102.25
Accrued : 407.6086957
NPV : 102.2907609
Yield : 2.247124772
Mod duration : 8.809360184
Convexity : 0.8778663256
PV 0.01 : 0.0901116156
CashFlow: 
February 15th, 2014 : 1.25
August 15th, 2014 : 1.25
February 15th, 2015 : 1.25
August 15th, 2015 : 1.25
February 15th, 2016 : 1.25
August 15th, 2016 : 1.25
February 15th, 2017 : 1.25
August 15th, 2017 : 1.25
February 15th, 2018 : 1.25
August 15th, 2018 : 1.25
February 15th, 2019 : 1.25
August 15th, 2019 : 1.25
February 15th, 2020 : 1.25
August 15th, 2020 : 1.25
February 15th, 2021 : 1.25
August 15th, 2021 : 1.25
February 15th, 2022 : 1.25
August 15th, 2022 : 1.25
February 15th, 2023 : 1.25
August 15th, 2023 : 1.25
August 15th, 2023 : 100