CRM-3476 coma's in amount requested field not processed correctly

Numeric values entered in CiviGrant 'Amount Requested' field are not processed correctly if they contain a coma.

To recreate:
1. Create a new Grant
2. Enter amount = 1,324.89
3. Save
4. Observe that the saved value is 1.00

Desired behaviour is to mirror the value entry treatment in CiviContribute. (e.g. 1,324.89 is saved as 1324.89)