Add ability to capture output

Added ways to capture output associated with a context or vow where it was generated. An optional switch wraps stdout and stderr, or tests can directly write to VowsParallelReporter.output['stdout'] or VowsParallelReporter.output['stderr'] as needed. XUnitReporter puts this data in the system-out and system-err nodes.

Also, fix bug in xunit reporting when a vow for a captured error fails.