Implement a deep-equals method for content

[rolfl]

All JDOM content items should be able to perform a deep-equals with another content, and return true if the two elements, and all their descendents, are equals too.

Comments should be ignored, and whitespace normalized according to standard XML rules (including where whitespace is significant xml:space="preserve"

Anticipated problems/complexities:

• whitespace
• difference reporting - is true/false enough?
• unused namespace declarations

See: http://stackoverflow.com/q/17382199/1305253