Replace group separator character with blank string

[stamminator]

...instead of with a . symbol.

Fixes #12.

[stamminator]

Despite what Travis is reporting, it looks like my changes are failing the unit tests.

Chrome 73.0.3683 (Windows 10.0.0) ngInputCurrency ngInputCurrency should first update the model once the element is blurred. FAILED
        Expected 12350 to be 123.5.
            at UserContext.<anonymous> (C:/repos/ng-input-currency/test/ng-input-currency.js:109:27)
        Expected 12350 to be 123.5.
            at UserContext.<anonymous> (C:/repos/ng-input-currency/test/ng-input-currency.js:114:27)
Chrome 73.0.3683 (Windows 10.0.0) ngInputCurrency ngInputCurrencyService .preformatValue should remove the CURRENCY_SYM if its in the value FAILED
        Expected '12345' to be '123.45'.
            at UserContext.<anonymous> (C:/repos/ng-input-currency/test/ng-input-currency.js:156:122)
        Expected '12345' to be '123.45'.
            at UserContext.<anonymous> (C:/repos/ng-input-currency/test/ng-input-currency.js:157:122)
Chrome 73.0.3683 (Windows 10.0.0) ngInputCurrency ngInputCurrencyService .preformatValue should change the GROUP_SEP to a . if the GROUP_SEP only exists one place and DECIMAL_SEP does not exists FAILED
        Expected '12345' to be '123.45'.
            at UserContext.<anonymous> (C:/repos/ng-input-currency/test/ng-input-currency.js:161:85)
Chrome 73.0.3683 (Windows 10.0.0): Executed 13 of 13 (3 FAILED) (0.026 secs / 0 secs)

I will try to figure out what's wrong. In the meantime @jstroem, if you feel like checking it out as well, I have a demo with my changes running at https://stackblitz.com/edit/angularjs-p7wcc8.