Ensure that new user installation runs smoothly

[jchang504]

1. We should probably have an onInstalled page pop up like some extensions, with some README-like information on how to use the extension and maybe direct the user to options (KeepTabs doesn't do that much until you've added some hotkeys).
2. We also need to consider how to make the extension work a) immediately upon install and b) after an update. I think the way b) works is the browser auto-updates when the extension isn't running; i.e. in our case when the browser is fully closed. That's okay-ish, although many users probably never fully close Chrome and might instead try manually updating/reloading -- which would break KT functionality until they reload their tabs, as we've always dealt with in development. a) also needs thought, because tabs currently open when the user installs KT will not have the content script, and so won't have KT active on them. I think there's a way to inject the content script into those pages for a seamless experience. Both these items can be explained away in the onInstalled page, but of course it's better to have it be automatic and seamless for the user.

[jchang504]

This and similar questions on SO shed light on the (re)loading issue and some possible solutions. They're not very clean though. :\