Fix replacement of values in kwargs

[cgrtrifork]

Related to issue #36.

Check if the value is a string, an iterable, or other, in that order (order is important, as a string is also an iterable). Then do the replacements.

>>> from stackapi import StackAPI
>>> site = StackAPI('stackoverflow')
>>> tag = 'java'
>>> period = 'month'
>>> site.fetch('tags/{tag}/top-answerers/{period}', tag=tag, period=period)
{'backoff': 0, 'has_more': False, 'page': 1, 'quota_max': 300, 'quota_remaining': 300, 'total': 0, 'items': [{'user': {'reputation': 8583, 'user_id': 10819573, 'user_type': 'registered', 'profile_im ...

[AWegnerGitHub]

Thanks for the PR :) It looks good. I'll get it merged.