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.
(Sorry I wasn't done, my laptop shut down while reviewing)

I'm also not sure where these strings come from. Are they coming from the extension API? Are you building them somewhere else? (I suspect so since they don't exactly match ExtensionInstallPrompt::PromptTypeToString return values) It would be good to add a comment for that as well.