Fixed #12

Image.Save(string) apparently locks the file until the image is disposed
of, so instead save it to a stream that we have control over
Only mkdir when the upload fails

This should make regular uploads a little bit faster
Fall back to Renci.SshNet without WinSCP

Because let's be fair, requiring WinSCP.exe to be installed or shipped
with Superscrot is stupid.
Save screenshots of failed uploads (fix #14)
Misc. fixes

- Add XML documentations where missing
- Implement IDisposable on TrayIcon
- Remove shameful comments
- Remove unused, unfinished SettingsDialog
More XML documentation changes.