Add next and previous item methods

[jeremylynch]

I have added two more useful methods to move through lists.

item.next_item # returns next item in the list.
item.previous_item # returns previous item in the list.

Readme and tests have been updated and are passing.

Thanks

[pyromaniac]

Specs failed. Also why can't we use something like item.lower_items.first and item.higher_items.last?

[jeremylynch]

Apologies about the late response. It appears find_by is not supported by mongoid 2, so the specs weren't passing, I have fixed this issue by using a where().first instead. The specs should now pass.

As regards using item.lower_items.first and items.higher_items.last, it's more convenient to use item.next_item and item.previous_item. After all, this gem is about ordering documents, so traversing from one to the next should be convenient, would you agree?

[jeremylynch]

Ok finally I have worked it all out. The tests aren't passing due to an unrelated test and compatibility with mongoid 4 (you will see that the current build is also failing), i'm not exactly sure what is going wrong but it is worth checking out the Travis CI build. Anyway, the tests for this pull request are passing and it's ready to merge

[jeremylynch]

Thanks to @pjkelly for his pull request #21, the tests are now passing. This pull request is ready to merge.
What are your thoughts on merging this commit over @pyromaniac? Thanks

[pyromaniac]

Merged, thanks!