implement untested neuron network
implementation of external managed memory
debug: typo
debug: typo, end of namespace
debug: public constructor
Merge branch 'dev/nn' into dev/r-pkg
debug: implement sigma
Merge branch 'dev/nn' into dev/r-pkg
[feature] implement R interface of Neuron Network
Merge branch 'r-pkg' into dev/r-pkg
[Feature] Implement R interface for nn

[TODO:] segfault...
add standalone test program
Merge branch 'master' into dev/nn
include cstring
add standalone test program for nn
include stdexcept
use cmake to test
Merge branch 'master' into dev/nn

Conflicts:
	test/Makefile
add test of neuronnetwork
correct gradient