Raise exceptions on incorrect hook definition

[gaeljw]

Aims to fix #60

If an incorrect hook is defined and this cannot be caught at compile time, then an exception is raised at runtime with a message similar to this:

Some hooks are not defined properly:
 - IncorrectClassHooksDefinition.scala:11 (BEFORE)
 - IncorrectClassHooksDefinition.scala:14 (BEFORE_STEP)
 - IncorrectClassHooksDefinition.scala:17 (AFTER)
 - IncorrectClassHooksDefinition.scala:20 (AFTER_STEP)

This can be caused by defining hooks where the body returns a Int or String rather than Unit.

For instance, the following code:

  Before {
    someInitMethodReturningInt()
  }

Should be replaced with:

  Before {
    someInitMethodReturningInt()
    ()
  }

---

This PR also include a small refactoring of the GlueRegistry to use immutable types with private var rather than mutable types with public val. The former avoid to leak a mutable type outside.

[mpkorstanje]

This is really cool!