Tastyworks

Added
- Option model to store information about individual option legs
- Ability to execute options successfully (the last missing bit was option legs)
- Added first set of unit tests.

Added
- OrderDetails object to encapsulate order-specific details

Changed
- Internal changes and refactors to the orders model
- Re-architecture of the API _modus operandi_. Details in the README.md file.
- Moved the remote-fetching of objects (e.g. Order, TradingAccount) to their relevant object models.

Removed
- Trimmed down remote-fetching of objects from the TastyAPISession object, it is now a much more
light-weight object.

*NB: This is backwards-incompatible*

Added

Orders
- Class describing orders (Order)
- Ability to get existing account orders

Trading accounts
- Class for trading accounts (TradingAccount)

Changed
- Changed the session creation code, please see the new example file