fix sub-element bug and dry internal handling of options

[fledman]

in current master:

1. the[]= syntax does not properly update @sub_elements
2. the first call of the .= syntax does not properly update @sub_elements
3. every time you want to add a new option, you need to adjust each self.class.new callsite to pass the new option along recursively

this PR:

1. fixes the []= syntax
2. routes the.= syntax through []=
3. drys up sub-element creation such that each callsite only needs to specify the options it is overriding
4. removes the Ruby 1.9 backport

the external api should not change at all

[fledman]

@aetherknight some changes ready for review

[aetherknight]

I attempted to resolve a merge conflict on this PR using Github's online merge conflict editor. However, the tests were still failing. Instead, I made a local branch and fixed up the branch there.