Add Disable ESI Injection config option

[aricwatson]

Adds an option to disable ESI injection on specific blocks - use to override settings in turpentine_esi.xml without needing to edit the file and risk losing changes during an update.

For example:

 <reference name="header">
            <action method="setEsiOptions">
                <params>
                    <disableEsiInjection>1</disableEsiInjection>
                </params>
            </action>
        </reference>

Intended to fix #951