Support multithreading in the CPU mode of Solver::Solve

[kloudkl]

In each iteration of Solver::Solve, there are four chances to accelerate the computation.
The first opportunity is the most complex one since Net::ForwardBackward invokes the Forward and Backward of all the layers that comprise a net.

      Dtype loss = net_->ForwardBackward(bottom_vec);

The second chance is more straightforward. An OpenMP directive is enough to parallelize the independent computation for each param_id.

      ComputeUpdateValue();

The only extra trick that is needed to deal with the next occasion is to distinguish CPU and GPU mode.

      net_->Update();

The last one involves a plain old OpenMP friendly nested for loop.

      Test();

[kloudkl]

The codes do not have to be changed. Thanks to Michael Rutter, multi-threaded OpenBLAS package is available on all the versions of Ubuntu since Precise (12.04).
Multi-threaded OpenBLAS backported to recent Ubuntu releases
Follow the steps to take advantage of the powerful acceleration:

sudo add-apt-repository ppa:marutter/rdev
sudo apt-get update
sudo apt-get install libopenblas-base

Benchmark results are demonstrated in the related issue: #16

[Yangqing]

ComputeUpdatedValue() is not a big issue when doing large networks. It is the ForwardBackward() function, and the individual layers that takes the most time. Thus, parallellizing it will not give us much gain.

[sguada]

Within the ForwardBackward() computation, the convolutional layers are the ones which take most of the time (see #83) therefore parallelizing the loops there will be the most effective