Attempt #2 to cause yum to fail on invalid url
Spiff up the 'Developing Plugins' page

* Add more cross-documentation references
* Fix some broken links
* Add links to github/source code references
Fail yum on invalid url with .rpm suffix
Removing colon from example failure message

The example failure message was given as - fail: msg="Bailing out: this play requires 'bar'"

The colon in the example causes a YAML compile error, so I am replacing it with a '.'
Merge pull request #5665 from Taytay/devel

Colon in example failure message causes a YAML compile error
Fixes #5661 Handle null values for sysctl parameters
Make synchronize module work better with local transport, fix #5668
Merge pull request #5586 from tbielawa/spiff_up_plugins

Spiff up the 'Developing Plugins' page
fix str format error due to missing '%' in sysctl module
Merge pull request #5672 from simonz05/patch-1

fix str format error due to missing '%' in sysctl module
Merge pull request #5669 from mscherer/fix_synchronize_local

Make synchronize module work better with local transport, fix #5668
Correctly compare values as returned from 'sysctl -e -n'
correctly compare the values, better func name
Mark stat module as supporting check_mode
update default EC2 exclude regions -- this should be the best configuration for most users, but some may still want to tweak.
Update changelog
Merge pull request #5676 from simonz05/bugfix-sysctl-multivalue2

Correctly compare values returned by 'sysctl -e -n'
Merge pull request #5333 from mvermaes/yum_fix

yum module: Attempt #2 to cause yum to fail on invalid url
Proposing fix for Issue #4324; adding support for su in connection plugins ssh, ssh_alt

Fixes for ssh_alt support, adding in references to in_data where appropriate
Make sudo and su command line arguments mutually exclusive