Significant digit fix

[MegaphoneJon]

CiviCRM now stores 9 significant digits in the currency field, which means that Simple Donate displays amounts that look like: $20.000000000.

This PR does two things:

• Drops the numbers after the decimal point if the amount is a whole number (e.g. $20);
• If the amount is NOT a whole number, rounds to two significant digits (e.g. $12.50).