[Documentation]: fixed missing seeding option in PyGMO population
Merge pull request #46 from YS-L/doc/pop-init

[Documentation]: fixed missing seeding option in PyGMO population. In addition to sphinx documentation update, this patch provides a wrapper over the boost generated population.__init__ for clearer docstring. The wrapping population.__init__ still supports two existing methods of population construction, namely construction given a problem or copy-construction given another population.