Add skeleton testing framework. Fixes #21

This adds a framework for testing testdat with testthat. To run the tests, you would do:

```
library(devtools)
test()
```

Note that I moved the data files from `/local` to `/tests/testthat/data`, since I figured they're used primarily for testing the package. But if you want them to be available to users who install the package, it would be better to put them somewhere in `/inst` instead.

Other notes: 

* The `test_utf8` function returns TRUE even for non-UTF8 encoded strings. For example, you can see when you run the tests that one of the tests fails, because it expects `test_utf8` to return FALSE for a latin1 encoded string. If this is the expected behavior, then perhaps it's better to rename the function to, say, `test_nonascii` or something like that.