Various Installer fixes

* Renamed README files to enable markdown rendering
* Fixes adobe/brackets#7482
* Resolve half of adobe/brackets#3578
* Resolve half of adobe/brackets#1662
* Use `[ProductName]` instead of "This application" in "XpNotSupported" message

### Notes/TODO ###

* The message in the [first half](https://www.dropbox.com/s/lrf94ul163onl44/MessageUseDVDDrive.jpg) of 1662 is fine already. Really that show be throwing an "Unsupported disk" error instead of invalid character error, but that is a WiX issue.
* The "Canceling..." message is not even present in `WixUI_en-us.wx`. Without the message ID, it cannot be translated.
- [ ] Update some message (not sure which) for adobe/brackets#7678
- [ ] Critique message for Error 1327
- [ ] Error 1327 needs a French translation

I don't have the build system setup on my laptop yet, but everything should still compile.