Tighten up some options.Model/Sequelize/getModel related code.

[AndrewJDR]

Firstly, some background:
feathers-sequelize allows for overriding of its getModel method
in order to let users implement their own model provider schemes;

Overriding getModel() in this way allows for a different Model to be programatically
provided any time getModel() is called.
This can allow for multi-tenancy setups where different customer models/databases
are used depending upon different criteria, such as what hostname they're connecting to,
where they are connecting from, or whatever the programmer desires.

Currently, the feathers-sequelize Service class requires that a single Model be provided
in the options passed to its constructor. The problem is, when using a getModel() override
as described above, a single Model is probably not known at the time that the feathers-sequelize
Service constructor is called.

Therefore, a Model probably shouldn't be required at construction time.

This changeset makes providing a Model optional, adds an alternative option attribute called 'Sequelize'
for providing the Sequelize class (which is currently used to derive the default Ops), and
adds some warning messages inside of the Model getter to dissaude usage of it altogether other than
for coverage tests, since using this getter doesn't allow for proper getModel() overriding.

[AndrewJDR]

Side note, I'm actually wondering whether it's a good idea to have the .Model way of accessing the model in the first place?

It creates a problem for extended service classes that override getModel() to do more interesting things with providing different models under different situations based on parameters, so maybe it's better if a "parameter-less" version of getModel (i.e. the .Model getter) just didn't exist at all...

[daffl]

Thanks. I did remove the getModel but many applications still relied on it. It probably should be removed in the next major version. Also removed the console.warn since we got bit before by logging things that you can't turn off.

[AndrewJDR]

Thanks. I did remove the getModel but many applications still relied on it. It probably should be removed in the next major version. Also removed the console.warn since we got bit before by logging things that you can't turn off.

@daffl just to clarify, you mean that .Model will be removed?

[daffl]

Yes. But it will have to be another breaking change.