Initially integrate google test

This is a PR to track the work by @fergalbyrne ( #123 ), @rhyolight ( https://github.com/rhyolight/nupic.core/tree/google-test ) and me on  #10 .

I've confirmed that @fergalbyrne 's #123 was using gtest 1.7 by `diff -r` and this PR recreates the git history to be more clear. It's based on his work and with modifications of:

* also add `libgtest_main.a` to fix a bug reported [here](https://github.com/numenta/nupic.core/issues/10#issuecomment-47153233)
* relocate gtest headers, so it would be the same as the convention of other external libraries
* provide binaries for linux64
* add gtest license
* not including darwin64 binaries yet, waiting for @rhyolight to compile them and include

The commands are like:

```
svn checkout http://googletest.googlecode.com/svn/tags/release-1.7.0 googletest
cd googletest
cmake .
make
cp -rf include/gtest $NUPIC_CORE/external/common/include
cp -f libgtest*.a $NUPIC_CORE/external/linux64/lib/
cp -f LICENSE $NUPIC_CORE/external/licenses/LICENSE.googletest-1.7.0.txt
```

Not ready yet, please don't merge yet.