Add option for natural sorting and make it default, fix ascii sort

Adds an option to choose between natural sorting and sorting based on the ascii table, with natural sorting as the default.
Natural sorting just uses the natural sorting done by the `search` function.
Ascii sorting had a bug when multiple parallel namespaces only differed by similar numbers at the end, because the `:` char is behind numbers in the ascii table. This is fixed by replacing the `:` with a `/` char for sorting purposes.
Unittests and Travis integration for the above have also been added.

Further the signatures of the `render` and the `handle` function have been adjusted to the dokuwiki specifications.

A remaining possible issue: In the ascii sort pages are not sorted behind namespaces.