Register printers with replace option.

This makes development easier because now an updated version of the
printers just replaces the old version.
Fix name clash problems with intrusive iterators.

BoostIntrusiveListIterator and BoostIntrusiveSetIterator were used
for both pretty printers and internal helpers.

Cleaned up formatting (no more camelCase).
Test code for boost::intrusive::set and boost::intrusive::list.
Merge pull request #13 from johanst/intrusive_fix_iterator_name_clash

Fix name clash problems with intrusive iterators.
Merge pull request #14 from johanst/register_with_replace

Register printers with replace option.
Merge pull request #15 from johanst/intrusive_tests

Test code for boost::intrusive::set and boost::intrusive::list.