Ipay88: Use the amount in dollars instead of cents to generate the signature

Fixes the signature hashing to correctly use the amount in dollars with decimals removed, instead of the amount in cents. This gives the correct transformation that Ipay88 described with these examples:
$0.90 => 09
$1.50 => 15

Old (incorrect) behavior was:
$0.90 => 90
$1.50 => 150

@RichardBlair @jduff 
/cc @louiskearns