Add url parameter "private" to the loading api

The url parameter "private" allows users to set datasets as
private (not public) when loading them via the API by providing
a bool representation ("true", "yes", "1", "on"). This changes
the previous behaviour of the loading API in such a way that
datasets are now public by default.

The behaviour that the private setting of a dataset is only set for
new datasets, i.e. the private setting of an existing dataset remains
intact.

This also cleaned up a weird if <condition> return True else False
code in the ParamParser.
Update docs for the new loading api parameter (private).

Explain how the "private" url parameter, added in a previous commit,
works.
Update tests for the new loading api url parameter (private).

Tests now test for the public by default, private via parameter
set up of the loading API.
Merge pull request #809 from openspending/default-public-in-loading-api

Default public in loading api