Examples generate errors

One thing that R CMD check does is sequentially run all the examples in a package. If a single example fails, it reports the error and stops testing the examples. This is a useful test, and (I think) a reason to make sure that our examples do not generate errors.

However, it looks like the [last example](https://github.com/BAAQMD/strtools/blob/master/R/parse_integers.R#L17) for parse_integers is perhaps designed to fail. I handled this by wrapping the whole set of examples in a `\dontrun` block. But then it appears that the next function (str_relevel) has the same issue: the [last example](https://github.com/BAAQMD/strtools/blob/master/R/str_relevel.R#L10) throws an error. I can't be sure, but I think that this one is intentional as well.

At this point I decided to create this issue instead of wrapping the examples in a `\dontrun` or removing the last one. While I do think that having R CMD check run all the examples and report on errors is useful, it's possible you don't agree (or just weren't using / aware of this feature). If you'd like to use it, though, I recommend making sure that no examples generate errors.