nurse: if the is no repository, unistall all the plugin

[vincenzopalazzo]

It possible fall under a state (if you mess up with a buggy version of coffee) that you have the plugin installed but you have no repository.

So Coffee should remove all the plugins installed when there is no repository.

[tareknaser]

Good idea
We could even make the new nurse strategy more general in a sense that:

• For each plugin, check for the its remote repository
• We need the remote repository to exist so we can perform update for example
• If it doesn't exist, remove the plugin

Only concern I have is that we currently have 1 nurse strategy GitRepositoryLocallyAbsentStrategy and we are adding another one in #239. If we have too many nurse strategies, we check for too many things before each command run.
Might be a good time to make skip-verify a default.

What do you think?