Provide traceback up to the point of test failure.

busted -v shows the full tracebacks for test failures, but it was only
showing stack frames within busted itself. This patch uses xpcall to
capture the stack at the point of failure, so that stack frames in the
test file and it's callees are shown.
Don't show busted stack frames in tracebacks.

This patch filters out any stack frames from within busted itself from
tracebacks printed via 'busted -v'. It looks for the deepest stack frame
that is inside the test file, and discards everything after that. If no
stack frame exists within the test file then the error is within busted,
so the entire stack is shown.
_TEST_FILENAME: renamed current_test_filename and is now module local.
Merge pull request #196 from hellobeep/nice-stack-traces

Fix issue #192. Nice stack traces