MacGui: Add Turbo option for x264 two pass encoding

MacGui: Add Turbo option for x264 two pass encoding
- Adds a Turbo checkbox under the 2-pass checkbox in the video tab if x264 is selected and 2-pass is checked, otherwise the Turbo checkbox is hidden and deselected
- Appends ":ref=1:subme=1:me=dia:analyse=none:weightb=0:trellis=0:no-fast-pskip=0:8x8dct=0" to a given x264 option string on the first pass of a two pass encode if the Turbo checkbox is checked.
- in testing, roughly doubles the first pass encoding speed using the HB-AppleTV preset opt string on a 2 pass encode.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@607 b64f7644-9d1e-0410-96f1-a4d463321fa5