Create installation_centos6.md

This document details the steps for installing TheDataTank on CentOS 6. It includes a number of installation steps specific to the Red Hat family of Linux distributions, such as modifications needed to support PHP 5.3.3, the use of yum rather than apt-get, and configuration of SELinux and iptables.
Update installation.md

Added links to CentOS install document and packages for Windows support.
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Update installation_centos6.md
Update installation_centos6.md
Update installation_centos6.md
Update installation_centos6.md
Added links for CentOS and Windows install
New instructions for CentOS install
Merge pull request #22 from spikewilliams/master

New instructions for CentOS install by @spikewilliams, thanks!