Inject Agent and Page Extensions on native initialisation

Extensions need to be loaded before the page fires off it's own scripts, previously there was a call to injectAgent upon page initialisation, this seems to have been made lazy at some point. This reverts that change and thus makes extensions and the poltergeist agent available before the page runs it's own scripts...

I updated the integration test to prove this behaviour, squashed commits and also updated changes to fix some typo's from last time and added the new bug fix.