Add Criteria#<<(criterion) to stop repetition

[erran-r7]

Prior to this change Nexpose::DynamicAssetGroup#criteria accessor was used to get the DAG's criteria, Criteria#criteria was then used to get the actual array following Array#<<(criterion) which would be used to add a new criterion. Producing the longer syntax dag.criteria.criteria << my_new_criterion vs. my_dag.criteria << my_new_criterion.

Before

dag = Nexpose::DynamicAssetGroup.new('test123')
criterion = Nexpose::Criterion.new(Nexpose::Search::Field::OS, Nexpose::Search::Operator::CONTAINS, 'linux')
dag.criteria = Nexpose::Criteria.new

# We needlessly repeat criteria here
dag.criteria.criteria << criterion

After

dag = Nexpose::DynamicAssetGroup.new('test123')
criterion = Nexpose::Criterion.new(Nexpose::Search::Field::OS, Nexpose::Search::Operator::CONTAINS, 'linux')
dag.criteria = Nexpose::Criteria.new

# This is more straight forward
dag.criteria << criterion

[jhart-r7]

Agreed, this is much cleaner.