BOE (development version)

Update url to retrieve data from the API.
Now retrieve_sumario() provides informative error if a cve is provided instead of a date (#17).
New functions to access the supplements, suplemento_cve() and sumario_suplementos(). These are only accessible for 3 months.
New function valid_code() returns a logical value instead of an error as check_code().
New function last_date_boe() returning the date of the last valid publication.
Better handling of dates (new fecha_anulacion).
retrieve_sumario() keeps all columns even if they are empty.
retrieve_document() works for some documents before 2009.

Breaking changes:

If a function returns a valid CVE now it has an suffix _cve. This affects anuncio, disposicion and specially sumario*.