Module mediator

[akshay-ap]

Fixes #4 and partially addresses #6

Changes in PR:

• Add Mediator interface
• Add a contract implementing mediator interface
  - Users can add Mediator as a module
  - User can enable module through registry with root or non-root access
  - Contract emits events on successful execution or fail execution
  - Contract emits events on adding module for a safe
  - Validate if user asked to enable module with root/non-root acces
• Test contracts (example module and Test Safe account) for setup
• Test module mediator

Module mediator tests:

• Test enable module
• Test disable module
• Test mediator setup
• Test non-root tx execution from enabled module
• Test root tx execution from enabled module

Additional changes:

• Increase test coverage for Registry contract
• Add .solcover.js file to exclude contracts from test coverage

TODOs related to natspec will be handled later in #13.

[mmv08]

There is still pending action left - the to address restriction - but this can be implemented later. Good job sir

[akshay-ap]

There is still pending action left - the to address restriction - but this can be implemented later. Good job sir

created another issue here #23