added shouldDrawImages to DTAttributedTextView

- this was still missing after the change to lazily create the content view
- set both links and images drawing to YES by default
set image and link drawing to NO for demo
Some cleanup on DTAttributedTextView

- made classForContentView public and documented
- fixed problem where default values would not be set
- renamed internal setup method
Merge branch 'master' of github.com:Cocoanetics/DTCoreText