system_sysctl parameter for redis class; handles setting vm.overcommit_memory via 'sysctl' puppet module.

This commit adds a 'system_sysctl' parameter to Redis class.
This parameter is disabled (false) by default, but when enabled
sets the vm.overcommit_memory=1 sysctl setting on the Linux server.
In order to set this sysctl setting the sysctl module must be available.
The sysctl module required dependency was added to the Modulefile file.

The README.md file was updated to state how to use the 'system_sysctl'
parameter.

Some grammar changes.

ref: https://github.com/fsalum/puppet-redis/issues/18