Implement the TypeRegistry

[markstory]

This adds a static factory for Type mappers that behaves very similar to the one used with the ORM. My reasoning around this was:

• It provides a simple hook for ModelAwareTrait to use.
• It makes getting type instances much simpler as the end developer doesn't need to manage connections as much.

After this, I'll look into some bootstrapping code that will help automatically wire the plugin into controllers/shells, and wire up the FormHelper context.

[lorenzo]

It looks good apart from my 2 comments and the missing plugin files :)