add junit xml format option

discussion to add a junit xml output option:

usage:
phantomjs loadreport.js http://www.lemonde.fr performance junit

output:
<?xml version="1.0" encoding="UTF-8" ?>
<testsuites>
  <testsuite name="domReadystateLoading" tests="1">
    <testcase name="domReadystateLoading" time="78"/>
  </testsuite>
  <testsuite name="domReadystateInteractive" tests="1">
    <testcase name="domReadystateInteractive" time="0"/>
  </testsuite>
  <testsuite name="windowOnload" tests="1">
    <testcase name="windowOnload" time="125"/>
  </testsuite>
  <testsuite name="elapsedLoadTime" tests="1">
    <testcase name="elapsedLoadTime" time="128"/>
  </testsuite>
  <testsuite name="numberOfResources" tests="1">
    <testcase name="numberOfResources" time="29"/>
  </testsuite>
  <testsuite name="totalResourcesTime" tests="1">
    <testcase name="totalResourcesTime" time="1251"/>
  </testsuite>
  <testsuite name="totalResourcesSize" tests="1">
    <testcase name="totalResourcesSize" time="1344.237"/>
  </testsuite>
  <testsuite name="nonReportingResources" tests="1">
    <testcase name="nonReportingResources" time="0"/>
  </testsuite>
</testsuites>

drawbacks:
-use of 'time' attribute of junit for non-time value. exemple: totalResourceSize is not a time value of course...
-bad time unit even for time attribute: totalResourcesSize time=1344.237 and should be /3600

avantages: allow users to follow if performance increase or not across builds history (even if values are bad, you can see performance evolution

sample result in jenkins:
![capture decran 2013-08-23 a 16 09 27](https://f.cloud.github.com/assets/1169286/1016580/b2c080fa-0bfd-11e3-88cf-d0f5ffc10ae3.png)