Drop support for Python 2.4 and 2.5.

Replace deprecated 'zope.component.adapts' usage with equivalent
'zope.component.adapter' decorator.

Replace deprecated 'zope.interface.implements' usage with equivalent
'zope.interface.implementer' decorator.
Wrap ``with interaction()`` in try/finally
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Don't guess the content type with 304 responses which MUST NOT /
SHOULD NOT include it according to:
http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5

Unfortunately, the content type will still be guessed if the result is
set before the status.
Attempt 2 on python3 support
Infrastructure for py3 support
Partial python-3 support (work in progress)
Py3 effort in progress
All import exception fixed
Adding py3 compatibility, fixing tests
test_browserrequest passes
test_browserresponse passes
More tests fixed
Python-2 compatibility restored, paste.txt fixed
All tests pass
Fix support for Python-2.6
unittest deprecation warnings cured
More deprecation and other warnings fixed
Drop pypy support, since at zope.security dependency lacks support for it
Compatibility with python-3.3 added, support for pypy removed

py3-attempt2 branch merged