Add validation adding array nodes to Constraint

[ricardogama]

This PR improves validation of array nodes when instantiating or adding nodes to a Constraint object, since the following is currently allowed:

var constraint = new Constraint({
  foo: [new Assert().Required, 'foobar']
});

Although an error is thrown when validating against the constraint, the error could be previously thrown when instantiating it and with a more adequate message.

With such motivation these changes lead to the following behaviour:

• If an array node is provided with the first element's type not being Assert an error is thrown with the following message:

  Should give at least one Assert on array nodes

  This behaviour was already implemented but this message seems more adequate than the fallback message.

• If any element's type of an array node is other than Object, Assert or Constraint an error is thrown with the following message:

  Should give Assert or Constraint on array nodes

• If there is more than one element with Object or Constraint type on an array node an error is thrown with the following message:

  Should not give more than one constraint on array nodes

• The fallback error message for providing invalid nodes has been improved/fixed to:

  Should give an Assert, an Asserts array or Constraint

These changes do not break the current suite and tests were added for all cases, including a missing test for the fallback error message.