Added the habilitiy to block the use of the scientific notation 'e' on NumericUpDown.

This is not an issue, but a feature request.
I would like to block the user from using the scientific notation 'e' inside the `NumericUpDown`, so only numbers would be allowed.

I took the liberty to code it myself, so the PR is coming soon.
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### Environment
- MahApps.Metro **dev branch**
- Windows **7x64**
- Visual Studio **2015**
- .NET Framework **4.5**
