Berth

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* Made the "build" part of the configuration optional so you also can use Berth to just build Python and Ruby packages directly, since FPM supports that as input.

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* Enable the use of environment variables during build and packaging steps. This makes it easier to create an updated package by just increasing the version number one place.
* When a local path specified as a source for a volume doesn't exist, we now try to create a directory at that path and make use of that. If the directory creation fails then we complain like before.
* Minor improvements to configuration file checks.

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* Stop the package generation flow if the build script fails (exits with non-zero exit code)
* Disable hostname verification for Docker client, since the hostname and certificate doesn't match when using boot2docker - Fig does this as well
* Place the build script file in the current working directory of the host system instead of letting Python decide. It is only possible to mount paths inside ``/Users`` when using boot2docker on OS X
* Minor code changes in relation to Pylint code check

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* Initial release