Don't try to clone floats, nil, true, or false.
Add tests for cloning dates and times.
Add tests for cloning Regexps.

Includes a failing test; /regexp/.clone is not allowed normally, but
ruby-deepclone does permit this. Regexp.new('regexp').clone should
always be allowed.
Merge pull request #1 from antw/additional-classes
Include the RCLASS_IV_INDEX_TBL macro as it doesnt exist on the ruby includes.
Made it work with frozen objects; Added some debug helpers.
Check if the object is nil before setting an ivar. For some reason, this breaks the ||= operator.
Add support for Ruby 2.1

2.1 moves the iv_index_tbl attribute into rb_classext_struct defined in
internal.h. This meant that it was no longer possible to access it
directly using RCLASS_EXT. Ruby 2.1 will cast the RCLASS object into a
custom struct, from which we can access iv_index_tbl.