support reverse iteration in PaginatedList

This uses the 'last' and 'prev' links in the HTTP headers.
The usage is repo.get_issues().reversed
remove print #oops
bug fix: empty nextUrl if no next/prev

Fixes a bug introduced in the previous commits.
Now testing a case where no lastUrl is returned.
Also fixing a regression where nextUrl wasn't erased properly.
bugfix: returning real list instead of generator

Changed reversed(content) to content[::-1] in order to return
a real list and not a generator.
The code in PaginatedListBase.__grow does an iteration over
newElements when using `+=` and then returns them.
When using a generator, newElements would be empty when they are returned.
Update readme
Merge branch 'topic/ReversePagination' into develop
Make PaginatedList.reversed return a new list (follow-up #184)

This will avoid surprises if both the list and its reversed version
are iterated.
Update readme and doc for #184