Fix import errors

After trying to use python-snapchat using pip to install I found that I would get import errors.

`
>>> from snapchat import Snapchat
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "snapchat/__init__.py", line 1, in <module>
    from snapchat.snapchat import Snapchat
ImportError: No module named snapchat
`

On viewing the code downloaded via pip i found that the `__init__.py` file tried to import `Snapchat` from `snapchat.snapchat`, even though it was already in the correct folder and only needed to import it from `snapchat`. This change fixes the import issue and the examples works as expected.