Corrected 0 learning rate decay bug
Corrected 0 learning rate decay bug

Also renamed a parameter for the Adam Optimizer for consistency.
Documented the optimizers


Former-commit-id: ca3350c04302ac4e6c01cec6932aeabb7e5b5808
danielhers/dynet,chunyang-wen/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,chunyang-wen/dynet,clab/cnn,shuheik/dynet,clab/dynet,clab/cnn,xunzhang/dynet,clab/dynet,xunzhang/dynet,shuheik/dynet,chunyang-wen/dynet,xunzhang/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,clab/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,chunyang-wen/dynet,xunzhang/dynet,clab/cnn,clab/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,shuheik/dynet