Use _all__ correctly.

Expose 'PastebinAPI' and use lowercase '__all__' since is the correct
spelling of the import list.
Merge pull request #6 from ivanalejandro0/correct-use-of-__all__

Use _all__ correctly.
Add exception for post limit error.

Using this exception we can use the pastebin API without knowing the
error messages sent by pastebin.com, we can just trust the api to do the
proper comparisons in the PastebinAPI code.