Address uniqueness, ordering, properties

[hhaensel]

This PR relates to #150 and to #146

[ancapdev]

I don't have well thought out opinions about most of what's added here, but the global dict type override looks risky to me.

It can change the behaviour of a program in areas far removed from where it's set, which is only safe if you own and control all the code you're running, and even then it's hairy when things scale.

What's the motivation for having that a global setting, rather than passing in the dict type where you need it to be specific non-standard type?

[hhaensel]

I didn't want the PR to be breaking but I also wanted to offer a short syntax for returning OrderedDicts.
But I agree to your objection.

If you accept a breaking PR, my preferred solution would be to default to OrderedDict in all cases.
If you don't want a breaking change then document["key"] could return Dict while document[:key] and document.key would return OrderedDict.

[hhaensel]

Another question is whether we accept that document["key"] doesn't retrieve the last index. Otherwise we could fix that as well.
We could discuss whether for performance reason we always iterate twice or whether we always parse the first value and reparse the last if there is more than one entry. Unortunately there is no easy way to save an iterator's value without parsing. I tried bson_iter_value() and bson_copy_to(), but my trials failed.