Move various check and lint scripts from st2contrib to this repo

[Kami]

This PR moves various check and lint scripts from st2contrib to this repo. Those scripts are useful outside of st2contrib context and moving it here also makes it easier for StackStorm users to re-use those scripts.

This PR also cleans up those scripts a bit (adds consistent error handling, file names, etc.) so they are ready for consumption by other users.

TODO

• [x] Add section about st2sdk and those scripts to st2docs testing page
• [x] Update readme with some notes and examples of how we use it in st2contrib repo with our Makefile
• [x] Publish new version of st2sdk with those changes to PyPi - https://pypi.python.org/pypi/st2sdk
• [x] Update st2contrib repo to use scripts from this repo (for now, just rely on version from github)
• [x] Notify "interested parties" about this change

[Kami]

I will go ahead and merge this into master because I have some issues with pip cache on Travis.