jUnitReporter for integration with Jenkins CI

Hi Hadley

Please take a look at this attempt to make a Jenkins-compatible output from test_that. As mentioned, I needed to stably identify tests irrespective of whether they are passing or failing, and so modified expect_that to store the call itself. It also substitutes the "info" argument if given to differentiate such tests (typically in a loop). This works for expect_that but not for the wrappers like expect_equals etc.

To fit into the jUnit structure, context() becomes the `<testsuite>` name as well as the base of the `<testcase>` classname. The test_that() name becomes the rest of the `<testcase>` classname. The deparsed expect_that() call becomes the `<testcase>` name. On failure, the message goes into the `<failure>` node message argument (first line only) and into its text content (full message).

Execution time is also recorded.

I have tested with Jenkins on my local machine.

Best
Felix