Previously, URLObject.add_query_param was used to generate
next/previous page links in PaginatorMixin. This resulted
in (for example) page 2's "next" link having the params:
?page=2&page=3
Instead, URLObject.set_query_param should be used to replace
the current value of the "page" parameter.
Previously, generation of next/previous links would discard any existing
query parameters. This commit introduces a dependency on URLObject, which
is used to intelligently parse and modify URLs to ensure existing params
are preserved.
Addresses issues #107
