Enhanced implmentation of --dynet-gpu option in python end (#1152)
Enhanced implmentation of --dynet-gpu option in python end (#1152)

* Enhanced implmentation of --dynet-gpu option in python end

* fix copy list bug

shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,shuheik/dynet,clab/cnn,clab/dynet,clab/cnn,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,clab/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,shuheik/dynet,danielhers/dynet,chunyang-wen/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,clab/dynet,clab/cnn,danielhers/dynet,clab/dynet,shuheik/dynet,clab/dynet