Use correct data structure

Looping over a list to check a status code is not good behavior; instead use
a data structure that supports O(1) membership tests.

Thanks @Lukasa for the catch.
Remove duplicate check for isinstance

Thanks @Lukasa for the catch
from_int no longer needed

thanks @shazow for the catch.
Comment tests, remove pointless optimization
Surgery on errors

- Move them onto the class object so they can be overridden
- remove one-liner helper functions
- combine urllib3 errors, httplib2 errors
remove one-liner fn
Remove another inline function
Fix util documentation.
fix spacing
comment