Changing copyright headers from me to Google

When I initially released this project, I released it under my own copyright. I
have since then worked on it in my 20% time at Google (and want to continue
doing this) and my life becomes much simpler if the copyright is Google's.

From the perspective of how this project is run and managed, **NOTHING**
changes. YCM is not a Google product, merely a project run by someone who just
happens to work for Google.

Please note that the license of the project is **NOT** changing.

People sending in future pull requests will have to sign the Google
[CLA](https://developers.google.com/open-source/cla/individual) (you can sign
online at the bottom of that page) before those pull requests could be merged
in. People who sent in pull requests that were merged in the past will get an
email from me asking them to sign the CLA as well.