Expose validation metadata

The new Validatable.validations mixin method returns the validations as
an object
indexed by property name.
Applied Coobaha's PR fix - prevents undefined values
Validations configuration as object

Previously validations were appended to an array when configured. The
format was cumbersome, and led to issues. This refactors the
configuration into an object, as a property of the Model.

Note that if no validations have been configured, this property is
currently `undefined`.
Remove iteration of config args

Fixes #154 - args should only contain the property name at that point,
not loop over it.
Merge pull request #259 from fabien/fix/refactor-validations

Validations configuration as object