Config files to set global parameters

`.nupic_config` are files which contains important parameters related to the project.. They can be used to replace some environment variables which #813 couldn't remove them..

This PR is based on @utensil comment:

> Thus I prefer a dot file in .gitignore and developer could put it at the root of each check out of nupic, or (if we make cmake recognize), it could also be placed at ~ to take global effect. This is the same approach as we've seen in the .bowerrc of bower, the de facto package manager for the web front-end, quote:

> The config is obtained by merging multiple configurations by this order of importance:

In other words, when try finding a variable value, CMake could first search it at file located at root of the repo. If not found, it would search at HOME dir. And finally if not found, it would use default values.. Sounds like a recursive method to config nupic. Note that these files <b>don't need to be both exist</b>. Any of them could do. If both existed, CMake will follow the priority order mentioned.