codingstyle: trivial whitespace fixes

Reported by flake8.

tests: put all temporary files in one top directory

Use KALLITHEA_TESTS_TMP_PATH or a create a temporary /tmp/kallithea-test-*
directory.

Some temporary files might still be placed in /tmp ... but nothing should be
left behind.

vcs: better handling of invalid email addresses: don't consider them email addresses

13da89053853 was in principle right in always returning email adresses as
string ... but unfortunately the function also returned invalid email addresses
that didn't fit into strings.

To fix this, the function is refactored to always use regexp matching of valid
email addresses ... and to be simpler. The behaviour should be the same as
before for all valid email addresses.

tests: add test for author_email handling of invalid author strings

This shows that author_email can return invalid email addresses.

cleanup: use example.com for tests and examples

example.com is explicitly reserved for this purpose. Using that means
we won't accidentally hammer a real server or real email address if an
example value escapes into the wild, e.g. in an automated test.

The domain "kallithea.example.com" has been used throughout to refer to the
example Kallithea server.

remove vestiges of Python 2.5 support

We only support Python 2.6 and 2.7; hence we do not need to import
with-statement support from __future__.

Second step in two-part process to rename directories.
This is the actual directory rename.