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
        
We check for it to be > 0 because we can only set a key if there is no key already (I.e. len(keys)==0). There might be a way to make it clearer though; I'm thinking that self.keys might return none if there is no key available and that might be easier to read.

Since we inherit from the Targets object. Keys is updated when we call the super method.