GitLab

Update build badge to point to the correct location

point build badge back to proper repo path

Fix build icon location

Closes #587

Per the review comments.

Another AppVeyor move

Typos, punctuation & broken links

tiny-dnn in its recent version updated the testing framework to GTest.
We would like to be mentioned as consumers of the project.

Added a link to [gtest-runner](https://github.com/nholthaus/gtest-runner), which is a Qt GUI for gtest executables. It just had its initial release, and is being actively developed and tested on multiple linux and windows platforms.

On google code, the main documentation pages were the "Google Test Primer" content. Currently it's hard to find that content on github, so add a direct link in the main README.md.