Support IE 9 and lower better

Although it is already supported, the problem is that the back-end
expects 2 extra metaframes, if a stack trace was generated by us.

The first approach in `generateStacktrace()` provides these frames,
but if it fails, we fall back to the second approach, which lacks
them. It means that on the back-end side the parser chops off two
important frames.
test/test.bugsnag: refactor the IE 9 test

The problem with the previous test was that IE8 and lower don't run
it, because of the "addEventListener" check.
Change the format of the fake IE9 frames
Differentiate between IE9lte and other generated traces
Merge remote-tracking branch 'origin/ie9lte-support'