Core: Refactor how we load dependencies

This patch follow the discussion on mailing list
about the need of consolidate how we load our dependencies
in Deltacloud API.

This patch will remove *all* `require` from Deltacloud code
(except drivers) and require *all* dependencies on one place:

'dependencies_initializer.rb'.

Depending on how you use Deltacloud API, it will use Bundler or
fallback to system gems.
Core: Removed OrderedHash core_ext (obsoleted in MRI1.9)
Core: Added 'generate:gemspec' rake task

The 'gemspec' is not longer loaded in Gemfile.
For this we need to update deltacloud-core.gemspec
everytime we do a release (or doing `gem build`).
Core: Fixed dependencies for deltacloud-db-upgrade