Convert system tests to use rspec-system

This patch ports all of the existing system tests to use rspec-system instead.

To assist with this conversion some patches were made to fix OS compatibility
where necessary. We also added an ensure parameter to the postgresql::server
class to assist with removing PostgreSQL configuration to aid with testing
cleanups.

The documentation has been updated to indicate test usage with rspec-system,
we've also renamed the 'tests' directory to 'examples'.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #186 from kbarber/rspec-system-conversion

Convert system tests to use rspec-system
Add support for other operating systems besides Redhat/Debian

Prior to this commit, if you attempted to use the module to
manage postgres on any OS other than Redhat/Debian, there
was an explicit check for that, and a call to `fail`.

In reality, the OS family is only used to build up defaults
for various path and package names, which are all exposed
as parameters.  If the user is willing to explicitly pass
in all of those parameters, there's no reason we should
fail based on OS family.

This commit adds checks to the 'default' osfamily case
such that we now only fail if they're on a non-Redhat-or-Debian
system *and* they haven't explicitly passed in values for
all of the required parameters.