Merge pull request #250 from mbebenita/youtube

First sign of life from youtube.
Fixes regressions introduced by #55325c49a72 and #723c356f1c8
Undoing part of #93eb9d50829f61b3
Merge pull request #251 from mbebenita/youtube

Make interpreter cases use constants and optimize popMany.
Using systemXHR for URLStream in the inspector
Hack to execute _gotoFrame script during symbol construction
Refactors some 'public$' usages
Allows URLStream to POST data
Merge pull request #252 from yurydelendik/yt-stream

Initial implementation of sending POST data
Few youtube related AVM2 test cases
Assigns _syncCursor earlier in embed.js
Fix default values.