Easier unit tests and better examples with exampletestr and covr

In spite of the utility of unit tests, most R package developers do not write them. exampletestr makes it easier to start writing unit tests by creating shells/skeletons of unit tests based on the examples in the user's package documentation. When completed, these unit tests test whether said examples run correctly. By combining the functionality of exampletestr with that of covr, having ensured that their examples adequately demonstrate the features of their package, the developer can have much of the work of constructing a comprehensive set of unit tests done for them