Add Support for Checkstyle To gateway.server.api module

# Common Style Issues observed
* copyright header is not consistent with required code.quality, fixed to make comma optional
* trailing spaces inside the license text at the top of each file should be fixed globally
* many errors are simply trailing-whitespace lines; perl-pi.bak -e 's#(\s+)\n#\n#g' fixes most issues

# Decision to CheckStyle at Build Time
* checking at build-time is a good way to enforce standards, and they do not seem too burdensome.
* other Kaazing projects do this
* there is a design question about whether to use a local checkstyle.xml file for each repository (duplication) or whether to use a URL-based checkstyle.xml (introduces netowrk dependency to build).  I've opted for a local checkstyle.xml in these commits as the lesser of these evils, but I'm happy for anyone to revisit that decision.