Use gtest-1.7 instead of 1.6

I replaced gtest 1.6 with 1.7 based on current master branch, because @unnonouno 's patch(https://github.com/tanakh/waf-unittest/pull/5) conflicts with current master branch.

* removed gtest-1.6.0 directory
* unzip gtest-1.7.0.zip to gtest-1.7.0 directory
* replaced 1.6 with 1.7 in following files
 * README.rst
 * gen.py
 * unittest_gtest.py
 * unittestt.py
* executed gen.py

Please check this patch!

```
$ git diff 6a6b109f79dc3b117ed72dcfc6033525785331d2 5adc9e84bd62b287db58338790db35b67cb70886 -- README.rst gen.py unittest_gtest.py unittestt.py
```