dont use homegrown OrderedDict, use pythons standard

I was looking through the source for this library and I saw that you guys use Django's OrderedDict implementation which is now deprecated for pythons standard `collections.OrderedDict`. I took some time to changed all instances of `markdown.odict.OrderedDict` to the proper `collections.OrderedDict`.

1. Is this a good idea?
2. should I remove usage of OrderedDict from `api.txt` also?