Let's use De Factuur instead of Factr

De Factuur is the new Factr site, let's use that one by default
Don't send locale along as parameter anymore

The api mostly doesn't care about locale, as we're only getting data
from it, no user interface. If it needs locale, it has sane defaults.
Extract to method: Remove indexes from http array parameters

This is used in multiple places, so extracted to method.
The invoices call has a filter parameter, make it usable

Add the filter parameter to our api method
Allow sending a reminder for an invoice via the api
Allow getting a PDF invoice from the api

This required hacking the doCall method to let url's containing .pdf
pass through without checking their contents. This should be refactored.
Merge pull request #4 from sumocoders/feature-some-more-api-methods

Feature some more api methods