bidi: support for changes to bidi algorithm in Unicode 8.0

Code changes to support:
- updated rules X5a,X5b and X6a in Unicode 8.0
- max depth for nested brackets in Unicode 8.0
- updated definitions BD14 and BD15 in Unicode 8.0
- clarifications to rule N0 in Unicode 8.0

To makes the tests pass again:
- use par.getLevels() instead of using par.resultLevels directly in
  test file

Change-Id: I63bdba11d91932e62e830ce9b71b1a6c6261abb0
Reviewed-on: https://go-review.googlesource.com/29630
Reviewed-by: Sam Whited <sam@samwhited.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
3 files changed