Fitting result fixes

The intention of this PR is to change some of the functionality of `FittingResult`.

Its key additions/changes are:

- [x] Removes previous explicit `deepcopy` or `copy` calls from the fitting framework, i.e. from the method `fit()` (and subsequent calls) of all `(Multi) Fitter` objects.

- [x] Removes `view...()` methods from `(Multi) FittingResult` (although it remains a `Viewable` object, i.e. `_view()` is kept). Instead `(Multi) FittingResult` now returns images with attached landmarks (see methods `fitted_image` and `iter_image`).

- [x] Removes the property `error_type` from `(Multi) FittingResult`. Instead `error_type` is now a `kwarg` passed to the method `errors(error_type='me')`. This simplifies the signature of `fit()` methods since `error_type` disappears from its list of `kwargs`.

- [x] Removes the `fitted` flag from `(Multi) FittingResult` which was unused and probably unnecessary.

Note that, with these changes, `(Multi) FittingResult` has lost some of its functionalities (only in terms of visualizing the results). These should be brought back to `Menpo` by #412.

> This PR is placed on top of #411
why have we changed to deepcopy here? Seems `.copy()` would be preferred...