Add .clang-format settings file

[kastiglione]

I ran clang-format on the changes I made in #304, but it resulted in a few changes that did not match the existing formatting/style of the rest of the code. For example the indentation of public: and private:.

Adding a .clang-format file would solve this.

[allevato]

A .clang-format that's just BasedOnStyle: Google should be sufficient here, as long as public clang-format does the same thing as ours internally.