Add devtools support (boilerplate reduction)

I finished the initial version of [devtools](https://github.com/mbj/devtools). This pull request is against the least complex gem we have in the dm-2 stack. If you accept here I'll prepare pull requests like this for all dm-2 repos.

Had to split the ```Gemfile``` in to a project specific and a shared part. The shared one is ```Gemfile.devtools``` that lives in the project root. A specific rake task ```devtools:sync`` allows to copy the Gemfile.devtools on updates.

## Pro

All the boilerplate under ```tasks/**/*.rb``` is now sharable between projects. No development flow changes are required. ```git clone && bundle install``` still works. No submodules are required. Heckle is now configured with ```config/heckle.yml``` will add support for adding aliases here before doing a pull request on veritas.

All tools or infrastructures like gemnasium, travis, etc do not see any difference. Only the Gemfile is a littlebit fancier than before.

All changes to the Gemfile.devtools can be done and tested locally without the need to modify my devtools repo. Plan to give @dkubb, @solnic, @snusnu access to the devtools repo to share fixes.

## Contra 

We have split gemfiles and no explicit development dependencies in .gemspec anymore. The latter is IMHO not a problem.