Implemented unhandled exception handling

Hi,
I changed the CrashReporter a bit.
Open for discussion  :)

You can now use "**CrashReporter.EnableGlobalExceptionHandling(bool)**" to enable global exception handling. This will create a CrashReport as soon as a UnhandledException occures.

Other changes:
- On a crash, the messagebox will only be shown, if the application is not running in console mode.
- Existing CrashReports are no longer overwritten, instead the new report will be appenden to the existing.
- The CrashReporte filename can now be changed by the caller.