(petitioner divorced spouse and separation agreement provided for joint custody, said nothing about allocation of tax deductions, and listed ex-spouse as custodial parent; petitioner claimed HOH filing status, dependency exemption deduction, child tax credit, and earned income tax credit on account of petitioner's two minor children; IRS denied deductions, credits and HOH filing status; court disagreed with IRS, noting that I.R.C. Sec. 152(e) lists circumstances where noncustodial parent can claim child and, under statute, the issue is which parent had custody of child for the greater portion of tax year; "custodial parent" defined as parent with whom child resides for greater number of nights during calendar year; petitioner proved that her two minor children resided with her more than 1/2 of each of the years in question; petitioner provided more than 1/2 of support for one child in each of the two years in issue, and more than 1/2 of support for other child for one year; one child proper age for both years and other child proper age for one year).