EC2 external inventory documentation

Document postgresql_db and postgresql_user modules.

Document that apt-cache=yes is equivalent to apt-get update.

Update index.html

Remove branch arg from git docs.

I propose this quote from the mailinglist should go in the "What People Are Saying" section.

Document sudo_user in playbook

Add link to setup module when ansible vars first referenced.

The "branch" arg doesn't work, users should use "version" instead
for specifying branches.

Add a cross-reference link to the setup module documentation, so
users know where to look to find more detail about what ansible
variables are available.

Fix missing colon in YAML example

Adding the documentation for 'install-recommends' to APT.