Properly encode IDN domain names for SRV lookups.

Before Twisted 12.3.0, the SRV lookup done for outgoing
connections succeeded if passed a unicode string with all-ASCII
code points. A recent change made the DNS code more strict, and
only byte string are accepted as domain name.  See
http://twistedmatrix.com/trac/ticket/6245 for details.

This change makes sure domain names are encoded to their ASCII
Compatible Equivalent (ACE) version before passing the resulting
byte string to `twisted.names.srvconnect.SRVConnector`, as per
RFC 3490.

Note that while connecting to servers with an IDN domain name
now works properly, authentication using MD5-Digest SASL mechanism
will fail until http://twistedmatrix.com/trac/ticket/5066 has been
resolved.

Fixes: #77.
Release Wokkel 0.7.1.
Added tag 0.7.1 for changeset 4d3066bc4034
Compile against Twisted trunk and pypy, too.

This also adds a pyflakes check step.
Make pyflakes happy.
Fix bad test comparing the empty string by identity.
Deprecate prepareIDNName.

C{unicode.encode('idna')} does exactly the same as the recently
introduced L{generic.prepareIDNName}.
Do proper fallback for Request.stanzaType.

If `stanzaType` is not passed to the constructor of
`wokkel.generic.Request`, it should default to the value in the class
variable, not `get`.

Author: ralphm
Fixes: #80
Disable pyflakes doctest checking.
Make pyflakes env setting global.