Fix futures parameters

[dhexus]

• begin_delivery parameter is missing
• If auto close is requested, set 'auto_close_date' to last trading date (settlement price) before begin of delivery
• What's the use of 'children'?
• What's the meaning of 'end_date'?
• What's the meaning of 'start_date' compared to 'first_traded'?
• Please store first day of contract existence and first day contract is traded
• 'expiration_date' is the last date the future is valid, i.e. the last day of the delivery period (not begin_delivery)
• 'notice_date' should be empty for (financial) futures (compared to physical forwards)

[warren-oneill]

Moved to #2. please stop duplicating issues.

• will add
• auto_close is optional but I can add it
• children is used for cascading see https://github.com/quantopian/zipline/pull/673
• end_date is the date where the product ceases to be tradable
• start_date is when the product becomes tradable, first_traded is the time the first trade takes place
• that is exactly what start_date and first_traded are
• will change
• notice date is automatically set to expiration date is none if given.