Recording payment truncates the amount after the comma (whether thousands or decimal separator)

When recording a payment for an event participant, if the Total Amount includes a comma as the thousands separator, the amount is truncated at the comma, such that $1,000 becomes $1.

Steps to reproduce on the demo site:

1. create paid event with fee of $1,200
2. register a participant and include payment of "1,200"

I only discovered this bug when I was recording another payment for a participant. In the screen for recording another payment, I added "0" as the fee amount, and the Net Amount filled as "1,000.00". This gave me an error because the Total amount ("1000") didn't equal the Net Amount ("1,000.00") until I made it exactly the same format with thousands separator and decimal (that's another bug that is simply annoying).

I've seen that truncation at thousands separators has been an issue in various sections of Civi (c.f. https://issues.civicrm.org/jira/browse/CRM-14011 ).

I'll note that we DO use CiviDiscount, but this is reproducible on the demo site that does not.

I list this as a minor bug, as there is an easy workaround, but it would be pretty major mess to miss this for a while and have thousands of dollars unaccounted for.