Only show notifications for supported events

Fixes the following:
1) We should only show notifications if we have a matching survey type.
2) We should only show notifications for extension install events (see Issue #71)

w/r/t extension install events, I'm matching against these:
https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/extensions/extension_install_prompt.cc&l=194

I'm considering ExtensionInstallPrompt::INSTALL_PROMPT, ExtensionInstallPrompt::INLINE_INSTALL_PROMPT, and ExtensionInstallPrompt::BUNDLE_INSTALL_PROMPT as install events and ignoring all the rest. I've never seen the bundle install prompt but presumably it is not too different from the others.
Done