[boost] [variant] Basic rvalue and C++11 features support

Hi, There is a patch in ticket #7620 that adds some rvalue assignment operators and rvalue constructors for Boost.Variant library. I saw no activity from authors of Boost.Variant Eric Friedman and Itay Maman for a long time, so if there will be no objections, I'll commit patch to trunk in the middle of November. Any suggestions and comments are welcomed. -- Best regards, Antony Polukhin