Capture Java exception stack traces in options("java.stacktrace"), fixes #11

E.g.:

> loadWorkbook("aFileThatDoesNotExist")
Error: FileNotFoundException (Java): File 'aFileThatDoesNotExist' could not be found - you may specify to automatically create the file if not existing.
> cat(getOption("java.stacktrace"))
java.io.FileNotFoundException: File 'aFileThatDoesNotExist' could not be found - you may specify to automatically create the file if not existing.
	at com.miraisolutions.xlconnect.Workbook.getWorkbook(Workbook.java:1197)
	at com.miraisolutions.xlconnect.integration.r.RWorkbookWrapper.<init>(RWorkbookWrapper.java:42)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
	at RJavaTools.newInstance(RJavaTools.java:352)