MAINT: Refactored target order methods

Refactored the target order methods to separate their logic from the
other order methods.

This makes order_target the only target method that calls order()
directly.

order_target_value is passed to order_target instead of order_value.

order_target_percent is passed to order_target_value rather than
order_value.

This simplified the code and decouples the logic of target orders from
the other order methods. This allows the target order methods to be
developed independently from order_value and order_percent.
MAINT: Removed unused keyword arg

Deleted keyword arg include_open_orders, it was left over from working
on handling open orders.
DOC: Add @humdings to contributors.