Refactor chart.BarVaR to use plot.xts

[erichung0404]

Core functionality

• Create bar chart of the first xts series(if there are at least two columns).
• Calculate VaR(Value at Risk) in different methods and add it to the chart in line type.
• Color of bars can be changed by show.greenredbars=TRUE. Positive returns are in green and negative returns are in red.
• Methods' names are listed horizontally.