Change Positions Level Exposure plot to track Gross Leverage plot

[justinlent]

@humdings It looks like we will want to update the Ticker/Positions Level Exposure plot to track Gross Leverage in the same way which you updated the Long/Short Exposure plot per this PR: #140

How easy/difficult do you think it would be to update the Ticker/Positions Level Exposure plot in a similar fashion?

[humdings]

@justinlent Where would I update this functionality? Is it just making pos.get_portfolio_alloc drop the cash as well? This will also need to work for both daily and intraday data I'm guessing.

[justinlent]

@humdings David, yep, I think it's as simple to drop the cash as well. Since we don't really support any intraday analysis (yet) in pyfolio, I think its safe to just get it working at daily frequency for now

[eigenfoo]

Looks like #147 resolved this!