terrastate doesn't work with Python 3

[beatcracker]

> python .\app.py

Traceback (most recent call last):
  File ".\app.py", line 154, in <module>
    app = TerraStateApi(__name__)
  File ".\app.py", line 137, in __init__
    for code in default_exceptions.iterkeys():
AttributeError: 'dict' object has no attribute 'iterkeys'

I've fixed this by replacing iterkeys with keys:

# add custom error handler
for code in default_exceptions.keys():
    self.register_error_handler(code, self.make_json_error)

I'm not sure how to make this fix backward compatible, so I just leave it here in case someone else encounters this issue.