Logging infrastructure + miscallenous improvements

[ppar]

This pull request adds some logging updates:

• Use of Go's native log package replaced with the drop-in https://github.com/sirupsen/logrus
• Informative and debug logging statements added, useful log levels set on existing statements (except for reporters/jira, reporters/sentry, uploaders/s3goamz, which are still untouched)
• Functions' error results tweaked to include context of where the error happened; each error is logged just once
• Parameters -loglevel, -logfile, -log-all-tasks, -run-once, -help added
• /var/log/mesos-complainer added in the containers (to facilitate attaching a volume from the docker host)

... and some documentation and toolchain updates:

• README.md edited for clarity -- all config options are presented in a table, all environment variables documented explicitly, the$PORT variable documented, some redundancy removed and details corrected
• Makefile, run scripts and .gitignore added
• Some ineffassign and linter complaints fixed

Since this is kind of a mixed bag, I have separate branches these at

• https://github.com/ppar/complainer/tree/add-logging-infra
• https://github.com/ppar/complainer/tree/update-docs-build-env

Both branches should merge cleanly into master, but for this reason they also don't document the new logging options in README.md, so to also get those updates, it'd be easier to merge this combined add-logging-infra-update-docs-build-env branch.