multi-threaded bp++ beta release

["Danke Xie" <danke@pubms.pku.edu.cn>]

Hi, everyone,

        I have added a multithread feature into bp++. It accelerates the training of recurrent backprop networks by about 40% on a
2-CPU PC. For more information, please visit http://www.psy.pku.edu.cn/danke/pdp

        -- Danke