multiple drafts support

[yasser-sobhy]

This is multiple drafts support for Draftsman.

Changes:

• has_drafts now has a :multiple option if set to true Draftsman::Multiple::Draft will be used instead of Draftsman::Single::Draft
• Draftsman::Multiple::Draft has has_drafts? method instead of draft?
• For multiple drafts, every update is considered a draft, no updates to existing drafts
• Draftsman::Multiple::Draft uses has_many association instead of belongs_to

To change:

• Edit readme.md to reflect new changes
• Review draft_publication_dependencies and draft_reversion_dependencies to make them work with multiple drafts or remove them if possible (actually I bypass them if :multiple option is true)
• Make sure new changes passes all tests or add tests for new functionality

[tvcam]

I've been waiting for this feature, when will it be released on 0.8 or merge to master?

[jmfederico]

Can you update the PR to resolve the conflicts?

[jmfederico]

This PR adds new functionality. Before merging or adding any new functionality, tests must be written for it.