Allow to customise the idSite in which the Piwik usage data will be recorded.
This will be useful to keep the piwik.org web analytics reports clean of the demo usage stats
Refs #5062 Disable PHP 5.6 tests as they don't pass:
`zend_mm_heap corrupted` https://travis-ci.org/piwik/piwik/jobs/24192025
 1) Test_Piwik_ServeStaticFile::test_firstAccessNoCompression https://travis-ci.org/piwik/piwik/jobs/24192027
Merge remote-tracking branch 'origin/master'