content for airco, security in datacenters

Filled some content for review
Added screen usage
converted action items to todos. removed duplicate lines
forgot an extra line
Merge remote branch 'upstream/master' into earthgecko
Added screen usage example, styling and s/bash/console/
Changed all '.. code-block:: bash' strings to '.. code-block:: console'
Modified:
config_management.rst
cron_101.rst
databases_101.rst
monitoring_201.rst
shell_tools_101.rst
sysadmin_tools.rst
unix_signals.rst
Removed console_output lexer
Merge pull request #85 from earthgecko/earthgecko

Added screen usage
Point people at github's git docs for now

We'll want to actually cover git and github in the curriculum, for sure.
For now, pointing people to the docs Github have already written should
provide at least some useful info. I've also pointed people at the IRC
channel if github's docs are insufficient.
Fix the tyop, thanks Mike
Merge pull request #91 from apeiron/topic/apeiron/git-github-docs

Point people at github's git docs for now
updates links for some redirects and remote vs local
whitespace removals
Merge branch 'master' of https://github.com/FlorianHeigl/curriculum into FlorianHeigl-master
Fixing warnings for list indentation
whitespace removals
datacenters 201: typos
Merge branch 'master' into topic/apeiron/sysadmin-tools-ssh