Double precision for solvePnPRansac()

solvePnPRansac() and pnpTask() now accept object or image points with double precision.
This PR is based on [PR 2948](https://github.com/Itseez/opencv/pull/2948) which was accepted to the 2.4 branch. Since auto-merging created conflicts, I resolved them manually and propose the integration into the master branch