Allow setting currency to blank value in both the settings and on the invoice.

[ficematt]

Allow setting currency to blank value in both the settings and on the invoice. This would allow the option to not show the currency on the invoice.
Current behavior is for the invoice to default to EUR when if delete the value and save the invoice.

The use of the currency label is not needed 99% of the time in the US. I know that's not the case elsewhere, but it would be nice if I had the option of leaving it assumed. Cleans up the invoice nicely.

[ficematt]

I did end up just removing the currency field form my template, but the behavior of putting EUR back in the field when you set it to blank is incorrect. At the very least it should set it to the currency that is set in the settings page.