Custom Predicates and `renameMany`

In order to impress @imjoehaines, a couple of functions have been added to better accommodate some custom use cases. Custom predicates can now be added via the must/should methods (for fatal and non-fatal predicates, respectively) in the ValidatorAbstract, and keys can now be renamed en masse using the renameMany function within the Assoc formatter.

Formatters

The library now has two component parts: the validators and the formatters. The validators haven't changed, although their root is now Schemer\Validator. The formatters work from Schemer\Formatter, and are documented in the README. If you have any questions, feel free to ask in the issues, and documentation will be updated :)

Initial Release

The repository has 100% code coverage, and is documented. At this point, the most likely thing to change drastically is the documentation.