Check coding style script

Create a coding style checker script inside t-e-c repository.

I think it's a good idea to have the checker script as part of the repository because (a) it's easy for developers to run it and test it   (b) it's easy to change the script to add exceptions.

For example, when adding XW_Extension.h in another patch, I would always fail the checker because it won't follow one of the rules (on purpose, the file isn't supposed to be modified). If the checker script was part of the repository, I could simply add the skip.

For this to work the trybot would have to use the script inside the t-e-c repository instead of an external one. This version is crude and doesn't check "only changes from external files". We probably can reuse the code from the existing script in build-infrastructure repo, but I couldn't find the license of it.