1st attempt at SIMD sigmoid (#1200)
1st attempt at SIMD sigmoid (#1200)

* 1st attempt at SIMD sigmoid

* Don't use fmin/fmax

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