imported from 3dptv.github.io the first page - introduction to 3D-PTV

In this pull request I have added the introduction part of the 3dptv.github.io using html2rest https://pypi.python.org/pypi/html2rest

$$ html2rest http://3dptv.github.io/wiki/an_introduction_to_3d_ptv/index.html > an_introduction.to_3d_ptv.txt

and manual editing of the file (not everything is translated properly). 

1. image 3dptv.png is added to images/
2. links are fixed manually like:`Turbulence Structure Laboratory <http://www.eng.tau.ac.il/turbulencelab>`_

The rest was rather straightforward.