Fixed duplicate instance bug with installer

Fixes issue #3

When the installer was launched without elevated permissions, it would start a second process of the installer while shutting down the current one. However, the constructor for the MainWindow was still invoked on the closed installer (I'm not sure why..), causing the installer's to clash when creating the settings file.

This pull request just adds a check in the constructor to see if the current App is closed or not before proceeding.

**Tested**
* Windows 8.1
* .NET 4.5