DLS method for solving PnP task + Documentation update

Added DLS method for solving PnP task in the calib3d module. 

Method based on the paper of Joel A. Hesch and Stergios I. Roumeliotis. "A Direct Least-Squares (DLS) Method for PnP".
I actually suggest to add SOLVEPNP_ prefix to ITERATIVE, EPNP, P3P and DLS. Otherwise it's becoming more and more dangerous in terms of name conflicts