fix(currencyFilter): Remove hardcoded fractionSize in currency

The hardcoded fractionSize in the `currencyFilter` makes the minFrac and maxFrac options in $locale redundant.

The default minFrac and maxFrac options in $locale are functionally equivalent and removing the hardcoded value would allow the behaviour of the currencyFilter to be customisable.

The motivation behind this change is to make i18n of currencies slightly easier.

Currencies such as the Yen typically are rendered with no decimal places, and the Dinar is usually rendered with 3 decimal places. Please refer to http://en.wikipedia.org/wiki/ISO_4217#cite_note-ReferenceA-6 for a full list of currencies and their decimal places.