Added DuplicateOrderedDict

[SleepProgger]

I added your DuplicateOrderedDict implementation as referenced in #5 , and some test cases.
Also, i made the usage feel a bit more like a "real" dict.

If you don't like my changes please tell me what to change.
IMHO, the vdf lib really need something like this build in asap.

[rossengeorgiev]

If we are to ship this we need to implement all the methods from https://docs.python.org/3/library/stdtypes.html#mapping-types-dict

Should be compatible with py2 and py3. For example, in py3 .values() returns a list, while in py3 you get a iterator. To get a iterator in py2 you need .itervalues(). It would be nice adhere to that API.

All that with the order/duplicates in mind, of course. At a glance it seems that it might not be the case currently.

May as well consider calling it VDFDict or something else as it's specific to VDF, rather than a generic solution. For example, one of the underline assumptions is that keys can only be a str or tuple.

Finally, might as well throw the code for it in a separate file as this is __init__ is getting long.

[SleepProgger]

I refactored the code a bit. It should now be compatible with py2 and 3.
Also moved the class to its own file.
Let me know what you think.

edit: We should def squash my fail series if you accept this pull request. (Sorry for the mess)

[coveralls]

Coverage decreased (-0.7%) to 99.346% when pulling 448a357 on SleepProgger:duplicate_dict into 8e50d5a on ValvePython:master.

[coveralls]

Coverage decreased (-4.2%) to 95.806% when pulling 14d84fe on SleepProgger:duplicate_dict into 8e50d5a on ValvePython:master.

[rossengeorgiev]

Hi, I've pulled your code and reworked it. See #8