Add a 'strict_callables' option to Mustache_Engine

Prevents treating array('ClassName', 'methodName') or array($instance, 'methodName') as section or interpolation callables. This helps protect against arbitrary code evaluation when user input is passed directly into the template.

This setting is recommended for PHP 5.3+, and all PHP 5.2 projects not using section or interpolation lambdas.

The default value is false to preserve backwards compatibility.

See #138