Registry upgrading

[pabloruiz55]

• All registry upgrades (STR, TR & MR) will be done in-place using a proxy / delegate call pattern.
• This means that anyone using these contracts (e.g. SecurityToken or Modules) has no control over whether they get the upgrade or not.
• As a result of this, any dependencies on these contracts are subject to breaking (i.e. if Polymath self-destructs the contract), which implies no existing functionality should have any dependencies on the registry contracts.
• AFAICT this is true for all of our modules and security token. The security token has a dependency on the module registry for adding new modules, but this is fine as it doesn't contravene the aims above which relate to existing functionality, not new modules.
• Registries do have interdependencies (e.g. module registry depends on STR, STR depends on TR), so when upgrading registries we'll need to make sure they are upgraded in a consistent manner.