rewrite installer, uninstaller + clean installdir

- Adds a welcome page, as it is the case in most installers
- Adds "license.txt" when PT is installed (instead of having it only displayed during installation)
- Remove some uneeded files, mostly '.*' (hidden) files and 'test', 'example' directories, as they are not needed to run PT
- Uninstaller now asks the user if they want to remove databases (clean completely all PT files if the user answers 'yes') - Sadly also adds a lot of strings for internationalization.
- Refactor the NSI script by removing hardcoded names and getting strings from parsing package.json instead (only hardcoded string is now '0.9.2' as in the node-webkit version used)
- Rename the uninstaller start-menu shortcut to "Uninstall Popcorn Time", to allow the user to find it with a search instead of having to manually locate it.