Switching to use Bundler in development environment.

This is a WIP set that targets to switch from using system gems
and ignoring Gemfile to properly use Bundler in our codebase.

With this patch, Bundler will be used when you start Deltacloud API
from the GIT repository (development env) or you run tests in GIT
repository (test env).

In case you use 'deltacloud-core' gem or RPM, the system gems will
be required instead of Bundler.

The last patch provide easy to use 'deltacloud-core.gemspec' generator.
This generator will update dependencies in 'gemspec' to match with the
Gemfile. So in future there is no need to touch 'deltacloud-core.gemspec'.