(court rejected Tax Court's finding that wife of taxpayer not bona fide employee for purposes of medical reimbursement plan; immaterial that reimbursements originated from joint checking account of taxpayer and spouse; taxpayer carefully followed rules for establishing employment relationship and deducting amounts paid under plan; court rejected IRS position that deductibility of medical reimbursements hinges on whether or not expenses might be paid from another source, even if that source has an obligation to pay, as lacking any support in caselaw; on remand, Tax Court is to determine whether wife was bona fide employee by applying common law principles of agency).