Use isinstance check so library can be used for more types

[svisser]

By using if isinstance(v, collections.Mapping) you can make this library work not just for dictionaries but also for the common dictionary types in the standard library (such as collections.OrderedDict) as well as subclasses made by the user:

>>> d = collections.OrderedDict()
>>> type(d) is dict
False
>>> isinstance(d, collections.Mapping)
True

[coveralls]

Coverage remained the same when pulling 63c0cd9 on svisser:patch-1 into ff9c7e3 on bfontaine:origin/master.

[bfontaine]

Merged, thanks!

[svisser]

Thanks for merging. I have also written a section at https://github.com/svisser/python-improvements/blob/master/README.md with some more details.

Have a good 2015!