* Improve exception class hierarchy
* Raise exception on malformatted "from_address" parameter instead of
yielding a negative verification result
* Add option to raise exceptions on ambiguous results
* Improve exception parameters to allow for a more structured analysis
of the negative response
* Yield a negative verification result upon the first permanent error on
RCPT TO, just like a mail server would also bounce after the first
permanent error
* Unify error message format, always include SMTP status code
* Use "with SMTP" to properly close connection even in case of error
* Establish connection after setting debug level so this is debugged as
well
* Properly handle SMTP error status codes on connection
* Don't return ambigious SMTP status codes, as they are not used anyway
Two essential changes were made to the process of initially downloading
the blacklist at install time, allowing the removal of a number of hacks
and workarounds:
1. Download the files *after* populating the build directory instead of
*before*. Thus, the files don't have to be included in MANIFEST.in, and
following from that, `setup.py sdist` needn't be tricked into not
including them in the source distribution.
2. Only import the updater module instead of the full validate_email
package, so `setup.py install` and friends don't depend on all the
requirements of the library.
