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.
Should the strings match ExtensionInstallPrompt::PromptTypeToString? If so, you might want to add a comment. Also add a comment that EXTENSION_OTHER is a catch all string rather.