Developer tools

== Summary 
Added the Developer tools:

The developer tool's and their documentation where added to aid project maintainers to generate and update the metadata for the projects they decide to host in a TUF repository. 


== Files Added
- Readme-developer-tools.md
    - the first draft of the documentation for the developer tools
    - describes the folllowing functionalities
        - creating a new project
        - adding/removing targets
        - adding/removing keys
        - adding/removing delegations 
        - writing metadata
- tuf/developer_tool.py
    - Added the project class, that inheritted from Targets:
    - Added the basic functionailties: (inheritted methods are not mentioned)
        - Creating new projects (create_new_project())
        - Loading projects (load_project())
        - Adding/removing keys (overriding the Target's class to force only one key)
        - Writing metadata (write() and write_partial())
        - Status(), which works as a --dry-run on the temp folder, this is expected to be updated. 
    - Added supporting functions for the ones listed above:
        - _strip_prefix_from_targets_metadata
        - _print_status
        - _generate_and_write_metadata
        - _save_project_configuration
        
Adding a comment that the parent Project() object is restricted to 1 key, and its delegated roles can have keys > 1.
Is this not fully implemented yet?  I doesn't appear `self.keys` is set anywhere.