Numbers > 0.50 should round up

In your spec, you have this expected to be '$1,234,567,890':

     NumberHelpers.number_to_currency(1234567890.506, {precision: 0});
 
Rails does this (IMO more sensible rounding):

    > number_to_currency(1234567890.506, precision: 0)
      => "$1,234,567,891"

The rounding of .50 up or down is a matter of opinion, but this is certainly a bug:

   NumberHelpers.number_to_currency(2.999, {precision: 2});

   => $2.00

And even this:

   NumberHelpers.number_to_currency(2.99, {precision: 0});

   => $2

This adds more specs and fixes the bug. I also round .506 up because in my 
personal usage of this library I want to match Rails's behavior.

Thanks for the library!