Added add_element and add_sequence methods

These are convenience methods for creating Elements and Sequences
belonging to a specific DObject or Item.

Instead of:
dcm.add(Element.new('0010,0010', 'John Doe'))
One can now use:
dcm.add_element('0010,0010', 'John Doe')

Obviously, the much simpler (but also somewhat slower) dynamic method
name approach is still available:
dcm.patients_name = 'John Doe'