Add CNMeM support using global MemoryHandler

[slayton58]

This is a different approach to adding a memory pool than my last PR -- this moves the pool management to a global singleton, independent of Caffe's singleton, to avoid any unintended side-effects of the thread-local storage used in the multi-gpu code.

Destruction of the memory pool's resources is now handled safely using an activator object (MemoryHandlerActivator) that initializes the pool on construction and finalizes it safely on destruction.

[lukeyeager]

This doesn't update the CMake build. Do we want to abandon that on our fork? I'd rather not.

Plus, I think we could pull in CNMeM from GitHub through CMake automatically. That would be neat.

[lukeyeager]

FYI, I typically use CMake to build Caffe with CNMeM support:

mkdir build && cd build
cmake -DUSE_CNMEM=True -DCNMEM_INCLUDE=/path/to/cnmem/include -DCNMEM_LIBRARY=/path/to/cnmem/build/libcnmem.so ..
make -j4