Using mapper=None for binary_loads and parse raises a TypeError exception

[pickworthi]

I found this when using appcache.py from the steam repository. For it's function parse_appinfo, it uses the default parameter specification of mapper=None. When this is passed to binary_loads it causes the TypeError exception, since None is not a dict type.

I think it is common practice to use None as a default for 'no value given - use default' in Python.

I will submit a Pull Request that fixes this fairly trivial matter - makes the code a bit more robust.

[pickworthi]

Submitted pull request: #4