Added some accessors to Dict
Added some accessors to Dict

cherryc/dynet,Abasyoni/dynet,Abasyoni/dynet,mitchellstern/dynet,kamigaito/cnn,Abasyoni/dynet,Peilun-Li/dynet,kamigaito/cnn,mitchellstern/dynet,cherryc/dynet,Peilun-Li/dynet,Peilun-Li/dynet,cherryc/dynet