Save client id on local database

[kosssi]

Actually we could save client information when we register an auth client. cozy/cozy-drive#113

It's better if cozy-client-js save it on local database to unregister the client easier.

[enguerran]

As getClient() is an API to GET /auth/register/:clientID with a registrationAccessToken required, it seems not feasible to set up a getClient() without any parameter. Thus it seems to be the third-party application's responsibility to store these two pieces of information (clientID and registrationAccessToken) needed for the request after a registration in order to get further accessibility to getClient() and others https://github.com/cozy/cozy-client-js API.