Splitting filtering logic from XML processing

Moved the filtering logic, currently restricting the processing to Java questions
with accepted answers and containing stack traces, from the XML reading
to its own class.

This gives the developper an easiest way to tweak or replace the current
filtering logic by extending the XMLEventFilter interface.
Fixed broken line number parsing

Surrounded frame line number parsing in try catch to avoid crashes
whenever the stack trace parser fails to properly detect the frame's
line number (either by leaving leading, trailing or in-the-middle spaces
or leaving some text at the end).

If the line number cannot be read, the default value is -1.
Added stack trace persistence in SQLProcessor

The SQLProcessor now stores the stack trace splitting it into a stack,
links and frames.

All the stack traces added to the database are linked, in the order
given by the parser, to the post they come from.