[Refactor] Update package to use camel-case field names

[lindyhopchris]

When this package was first being developed, the JSON API spec recommended dash-case (e.g. created-at) for resource member names. This recommendation has since changed, and camel-case is now recommended (e.g. createdAt).

This PR goes through the package an amends it to follow the most up-to-date recommendation of camel-case. There are actually very few implications on the package. These are:

• For the SoftDeletesModel trait, the default soft deletes field is now the camel-case version of the model column, not the dash case. Developers can easily continue to use dash-case by setting the softDeleteField on their adapters.
• The stub for the Eloquent schema now uses camel-case for createdAt and updatedAt. If the developer is using dash-case, it is exceptionally simple to amend those two field names once the resource is generated.

Because of the soft deletes change, this is a breaking change so will have to be released in the next major version (3.0).

Closes #393