Store session values in cookies.

Currently, we (or more precisely, beaker)  store session values in the filesystem.
That does not scale, and is extremely hard to synchronize across multiple machines.
Therefore, this code adds a new flag adhocracy.session.implementation that can be set to "cookie" to use a new session implementation which stores the session data in cookies.
The current implementation does not encrypt the stored data (as I haven't found any secret values), but does check a signature in order to prevent users from meddling with these values.
PEP8 improvements
use pytest instead of nose
Fix docs and CI script (use py.test)
Fix doctests

Use generator based test case behavior (this should work in pytest as
well as in nose) instead of `DocFileSuite` quirk, as the latter doesn't
work with pytest.

This is a bit of a hack (though not uglier then before) and should be
replaced by proper pytest doctest support.
build.sh: Remove unused variable (Never actually implemented)
Simplify and correct build.sh

build.sh should always work (=not need any options to do so), and be not overly slow on new installations (i.e. <30s).
Make installation work again. Probably due to confusion over the two different buildouts, installation failed (Fixes hhucn/adhocracy.hhu_theme#284)
In the future, we may want to simplify the
  system python -> local python buildout -> local python -> adhocracy buildout
chain, preferably to just one or maybe two links.
Remove buildout_offlinemode (-o). This should never have been merged, and was broken by definition (for one, it activated the non-superuser mode without any need to do so). This hhucn/adhocracy.hhu_theme#283
This also fixes an unrelated bug that always created a "1" file; we want 2>&1, not 2& >1.
Delete python/buildout.python/*pyc files in build.sh

Our change 0cfab874e3237c7270f9bfae6b0eb194d5b2b6c5 only works with git 1.7.12.3+ ¹, which is not standard on debian stable.
Alternatively, merging https://github.com/collective/buildout.python/pull/31 would fix the issue upstream.

¹ https://github.com/git/git/commit/8f6811efed5451c72aa
Merge branch 'develop' into cookie-session
Fix comment formatter (fixes #351)

This makes the comment formatter return *comment on xxx* instead of the
URL.

Showing the URL has been introduced in
afd15c9354ffdb06fdcfcda385a339bc66fe700c due to the removal of comment
titles.
Take comment.wording into account
i18n: update everything
Check for empty keys as well
Only use a single cookie for session data
Check session expiry in cookie-based sessions
Default session lifetime to an integer

If a year seems overly long, consider that github uses 10 years.
Clean up cookie session implementation
Correct setting and deleting of cookies
Merge remote branch 'origin/feature-343-pytest' into develop
Merge pull request #361 from phihag/cookie-session

Store session values in cookies.