Added a roclet with various test-related annotations.

Pretty much that.

@tests and @test are just like @examples and @example, expecting the following lines to contain test code or the path to a file containing tests, respectively.

@testthat and @context refer to the test_that and context functions in the testthat package.

Although I've set things up for the testthat package, it just copies everything after tests into a new file in "test/test-<filename>", so any other testing strategies should work just fine.