ENH: Add account object to context

Add an account object to context. This account object keeps track of certain account metrics. A complete list is below. These metrics may be referenced and used in the same way one would use context.portfolio. 

``` python
    account.settled_cash
    account.accrued_interest
    account.buying_power
    account.equity_with_loan
    account.total_positions_value
    account.regt_equity
    account.regt_margin
    account.initial_margin_requirement
    account.maintenance_margin_requirement
    account.available_funds
    account.excess_liquidity
    account.cushion
    account.day_trades_remaining
    account.leverage
    account.net_liquidation
```
When are these on / not on self. could you put a comment at the top of the function explaining why these are all guarded with default values.