Add support for stories

The current implementation was missing this feature, so I adapted the php-snapchat API for it. I also added an example of the usage of this API in the bin/ folder.

Additionally, I changed the import in pysnap/\_\_init\_\_.py from utils to pysnap.utils, thus allowing the package to function when installed to the system as a module. Previously, it would give an error about "utils" not existing.