Clean-up post-type registration
Add doc for Entry class
[WIP] Add validator class with initial capabilities to validate email addresses
Improve validation:

- Easy access to recieved data
- Easy access to passed/failed fields
- Small amends to other things...
Check if the field failed and output corrisponding data-attribute
Fix the textarea name not being outputted
Move Entry post-type creation to PostType class
Add passed() method and refactor the failed() method
[WIP] Make Entry() post-type CRUD operator
Structure change:

- Post types are configured more easily now
- Form() -> FormBuilder()
- Entry() -> FormEntry()
Add abstract class for all forms
FormBuilder() no longer requires an instance
Improvements to Validator():

- Remove Form related functionality
- Validator() now takes an instance of Form() for rules etc
- API imporvements