Allow config files to be organized in multiple directories

[jshaffstall]

I'm writing some packages that I wanted to host their own model and setting config files. The changes in this pull request expand the model_config_path and settings_config_path options to accept an array of directories. The directories are searched for config files in order, and the first match is used. This allows an application to override a package's config files if needed.

This is fully backward compatible, a simple non-array path will still work for both model_config_path and settings_config_path.

This pull request includes updates to the tests and docs for this change.

Also in this pull request, but not critical, is the Request::url () change to allow Laravel 5.2 compatibility.

[jshaffstall]

Some additional commit I made after I generated the pull request got pulled into the pull request, too. The last two commits are not about the config file changes, but are to allow the edit_fields section of the model config to be optional (I have a use case where I want custom actions but no direct editing of fields).

I hadn't thought those would get pulled in since I committed them after the pull request. Sorry about that!