Remove the need to specify an account number

[NelsonDane]

Right now, this API requires that you specify an account number at initialization and seems built assuming that people only have one account for trading. However, Tradier offers multiple account types. It would be more in the spirit of an API to allow people to pull their account numbers using a function instead of hardcoding them. The endpoint for that can be found here: https://documentation.tradier.com/brokerage-api/user/get-profile.

Let me know your thoughts, I'd be willing to put in a PR but wanted to discuss it first since it seems that a lot would need to be refactored.