decimal values passed as string now work (again)

[roadsideseb]

I am not a 100% sure that this is a fix for Oscar. I suggest this
change to be included because of the following reasons:

1. The previous implementation of the currency filter (pre-Babel)
   would return an empty string '' whenever an invalid type was
   passed in. It think it makes sense to be backwards compatible.
2. Applying the currency filter fails when passing in an empty
   string. This means that whenever the currency filter is applied
   it has to be ensured that it is not an empty value. Otherwise
   the filter raises an exception.
   This might cause additional issues due to the way the Djangoo
   templating system usually fails silently.

Please take a look at the suggested code in the commit and let me
know if this is a feasible addition/extension.

[roadsideseb]

Thanks for looking into it and sorry about the string thing. I must have overlooked that when I read your first comment.