Return topics in the right order

Passing -1 or n, where n >= num_of_topics to show_topics() should return the topics in the right order. It currently returns the topics in the reverse order which is difficult to reason about.

__Summary__
`lda.show_topics(-1)[0]` should return the same string as `lda.show_topic(0)`

I did not realise this bug until after going crazy for a week! :frowning:  There's probably a better way to solve this problem and I'm happy to update the PR if required.