ios videoplayer fixes

just finished a big commercial iOS project which really put ofxiOSVideoPlayer through its paces!
found and fixed a few bug which were lurking unnoticed,
and happy to say with confidence that ofxiOSVideoPlayer has never been more solid :)

here is a list of all the changes,
- fixed bug where seeking would sometimes get stuck.
- improved seeking accuracy.
- fixed crashes when accessing samples at the end of a video.
- fixed texture caching issue when video was recorded.
- added methods for audio sampling.
- added methods for enabling/disabling video and audio sampling when not needed. by default sampling is on.

since this was commercial project, the code was tested thoroughly across iPhone4 and up and iOS 5.0 and up.