Refactoring the refactoring :smirk:

I like what I'm seeing in this branch.

I thought it might make more sense to introduce namespaces for `Converter` and `Provider` and move the related classes into these namespaces, grouping together classes with related purposes. This PR moves the `Time` and `Node` providers into the `Provider` sub-namespace and the `Time` and `Number` converters into the `Converter` sub-namespace.

Additionally, I have added a `phpcs.xml` file for managing the coding standards rules I want to follow, and I've cleaned up some of the warnings and errors I found. I'd like to follow the [naming conventions](http://symfony.com/doc/current/contributing/code/standards.html#naming-conventions) implemented by the Symfony project, so I've added the "Interface" suffix to the end of interface names.

I've got some other changes I want to submit to your branch, but I'm holding onto them until you've been able to review these.

Thanks!