bring Exception::getTrace/getTraceAsString consistent with debug_backtrace

What do you think about adding the $options and $limit arguments to those
two Exception methods?

With debug_backtrace() adding those reduces the work the engine has to
do while collecting the data, getTrace() is simply passing a
pre-existing zval array to the user, so adding the logic their makes the
implementation more complex and usage of these options slows down the
operation (not that performance of those should really matter ;-) )

I'd prefer to keep it clean and simple :)

johannes