Adding function for GARCH model VaR backtest

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/uwgarp@135 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Adding vignette for quantifying Vol in VaR models

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/uwgarp@136 edb9625f-4e0d-4859-8d74-9fd3b1da38cb