Build for CPU only

Now, the minerva can build on CPU only.

I added SigmoidForward, ReluForward, and TanhForward CPU basic implementation.
For apps, the minst_mlp works fine.

To-do:
* other CPU implementations including conv, backward...
Spaces after keywords and curly brackets, as described [here](http://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Loops_and_Switch_Statements).