Handle expired API keys

[aazuspan]

Fix #62 by making the login and logout systems more resilient to expired API keys.

• A new USGSAuthExpiredError exception was added that is thrown when an "AUTH_EXPIRED" response is received. This occurs if a request is made with an expired API key.
• api.logout now does nothing if api.TMPFILE does not exist. This prevents logout attempts with an empty API key that would otherwise fail.
• api.logout now ignores USGSAuthExpiredError exceptions which would otherwise prevent it from deleting an expired API key. Other exceptions will still prevent key deletion, which ensures that a key is not accidentally deleted if it is not correctly logged out (i.e. if the request fails for a different reason).
• api.login now uses api._check_for_usgs_error to validate responses like the rest of the api module. This ensures that USGSAuthExpiredError (and it's hint to log out first) will be thrown if a login is attempted with an expired API key.

Let me know what you think, @kapadia. I'm happy to make changes :)

[kapadia]

Much appreciated @aazuspan

[kapadia]

All set, new package uploaded to pypi https://pypi.org/project/usgs/