Junit XML output #130

Whilst some of the CI systems will work natively with the default unittest-cpp XML output format, others just become a grind of fudges and XSLT. For the latter, a vanilla Junit format XML output would make everything seamless.

To solve my personal use case, I wrote an extension of UnitTest::TestReporter rather than deferred, mostly for two reasons: the first is that I simply couldn't get the deferred version to work reliably without core dumping on memory handling in the library (this is the Ununtu package, so a bit old and possibly down to build-time challenges too); the second is that Junit only accepts a single failure within a testcase, so there's no actual requirement to defer at all.

Anyway, the code works fine for me and I'm perfectly happy to contribute it back into the project if it would also save someone else some time and effort.

It has crossed my mind that this is an obvious extension to the library though, so there may be a very good reason that it isn't already there. Hence tagging this issue, so the maintainers have a chance to tell me to keep my scabby code to myself. ;)