Support for boost::thread and use of boost::async instead of std::async

The goal of this changeset is to enable parallelization when building Gaussian Process models without the use of std::async.
@arnaudgelas Could you check if the way I extended the boost build is reasonable? I am also unsure how libraries that are used by statismo should be exposed in statismo-config.cmake
@clangguth I haven't built it on windows yet. Could you check if it work with VS 2012?