added compute_balap function:

This will compute the balance after posting using this formula:

bal_beg = "Beginning Balance"
usd_post = "Total USD Postings"
php_post = "Total PHP Postings"
php_post_rate = "PHP Post Rate"
eur_post = "Total Euro Postings"
eur_post_rate = "Euro Post Rate"
bal_ap = "Balance After Posting"

bal_ap = bal_beg + usd_post + (php_post/php_post_rate) +
(eur_post / eur_post_Rate)