Support Profile URL for institutions like Vanguard

[eyal-feex]

Hi,
Vanguard has changed their OFX service so now you're required to call readProfile() before trying to fetch account data and then you have to use the cookies for the subsequent calls. readProfile should call a different URL than the rest of the requests.

I would suggest to add a profileUrl member to BaseFinancialInstitutionData and use that in readProfile().

Also, another change is required to keep the connection open between requests (and then close it when done).

[stoicflame]

Sure!

Want to put together a pull request for me to review?