Requirements: Provide a maximum acceptable release for Tastypie

Newer Tastypie releases do not support Django 1.4.x anymore.
Docs: Move 2.5.1 changelog to the corresponding release notes
Docs: Move 2.5.0 changelog to the corresponding release notes
Docs: Remove 2.1.6 from changelog

All data is already present in the release notes
Docs: Small cleanups

Partial cherry-pick for c4b627d8.
Remove stray os.close(fd)

A fallout from 2bc49e00d7485dabb5659c8104d5224ec1cad451.
Travis: --use-mirrors is deprecated in pip 1.5
Docs cleanups: Put all Fabric commands in a section

This increases readability on the index page, since this way no command
is being listed there.
Get TITLE and DESCRIPTION from siteconfig directly

Instead of changing django.conf.settings at runtime which we should not
be doing:
    https://docs.djangoproject.com/en/dev/topics/settings/#altering-settings-at-runtime
and is the source of bug 3150.
cache: Cache the top stats for a much longer time

Previous behaviour had us cache for x*POOTLE_CACHE_MIDDLEWARE_SECONDS
which is 10-15 minutes by default. That is ridiculously quick for
something that is very expensive to compute and doesn't change that
much.
By default we now cache for a whole day and this can be changed in the
new setting TOP_STATS_CACHE_TIMEOUT.
settings: Document POOTLE_TOP_STATS_CACHE_TIMEOUT
Docs: Advertise that LDAP auth backend moved in 2.5.1 release notes
docs: older pip might install pre-releases

Looks like pip might install e.g. 2.5.1-rc1 when 2.5.1 has been
released.  It would seem that their version detective work isn't working
in those cases.

This occured on Debian Wheezy (7.4) and pip 1.1

I don't think we want to remove the old versions from pip because we
want to allow fallback path for users if required and because these are
FOSS releases so we shouldn't just remove them.

So make a note in the install guide for this possible occurance.

Fixes bug 3185 to some extent.
skip missing units instead of crashing
add hook template for add-ons
create folder if not existing
add support for hint.html
add hook script for countries.json
update hook script for new html files
create LC_MESSAGES directory for contries.json