Populate model bound forms using form model accessor if set

[jonjrwhite]

This change enables use of Laravel's Form Model Accessors as detailed in the below documentation (documentation refers to 5.2, changes developed and tested in 5.3)
https://laravelcollective.com/docs/5.2/html#form-model-binding

The additional code uses the Form Model Accessor only if the method exists otherwise reverting to the original behaviour of using the standard model accessor.
This should mean that existing implementations are unaffected by the change.

The advantage of this change is that the value returned for binding a model to a form can be handled separately to the value returned by the standard model accessor. In my case I wish the value for the date attribute of one of my models to be formatted as a string when populating form fields, but to be a carbon date object for use in my reporting and metrics.

[coveralls]

Coverage decreased (-0.2%) to 94.55% when pulling 75502df on webhappens:enable-form-model-accessor into da9e21c on adamwathan:master.

[coveralls]

Coverage decreased (-0.2%) to 94.55% when pulling e1c965b on webhappens:enable-form-model-accessor into da9e21c on adamwathan:master.

[coveralls]

Coverage increased (+0.02%) to 94.787% when pulling 7527fef on webhappens:enable-form-model-accessor into da9e21c on adamwathan:master.

[coveralls]

Coverage increased (+0.02%) to 94.787% when pulling c0a5f6a on webhappens:enable-form-model-accessor into da9e21c on adamwathan:master.

[coveralls]

Coverage decreased (-5.9%) to 88.889% when pulling ab3ab61 on webhappens:enable-form-model-accessor into da9e21c on adamwathan:master.

[coveralls]

Coverage decreased (-2.1%) to 92.658% when pulling abc61f9 on webhappens:enable-form-model-accessor into da9e21c on adamwathan:master.