New methods findprev and findlast

While working on something else, I found I wanted a `findlast` method in analogy with `findfirst`.
Since `findfirst` is implemented in terms of `findnext`, I implemented `findlast` with the same
relation to a new `findprev`.

While working, I also noticed there were no tests for `findnext`, so I added a few.
I think there's just a lot of documentation that came into existence before there were (enforced) style rules. I know a lot of the Julia code I looked at when I started didn't have spaces after commas in argument lists, and while it did strike me as uncommon, it also seemed kind of "Julian" at the time. Notably, I don't find anything in the [style guide](http://julia.readthedocs.org/en/latest/manual/style-guide/) about which is preferred...