Add debug report (crash report) to Help menu

This captures crashes on Windows along with the stack backtrace.

On Linux (fedora 21 at least), the necessary function to enable
capture is not included in the system wx libs.  But, a self built
version works fine and capture the backtrace, so I'm assuming
other distros will probably work as well.

On OSX, the crashes are caught, but it does not include the
backtrace.

But, really, the backtraces aren't all that useful in a release
build since we don't ship with debug symbols and optimization
plays havoc with proper backtraces anyway.

The real benefit will be for the support folks as they can now
get consistent info from user by asking the to generate a report
from the "Help->Generate Support Data" menu item.

src/AudacityLogger.cpp

@@ -259,6 +259,13 @@ void AudacityLogger::Show(bool show)
    Flush();
 }
 
+#if defined(EXPERIMENTAL_CRASH_REPORT)
+wxString AudacityLogger::GetLog()
+{
+   return mBuffer;
+}
+#endif
+
 void AudacityLogger::OnCloseWindow(wxCloseEvent & WXUNUSED(e))
 {
 #if defined(__WXMAC__)