Use is_callable instead of method_exists
Use is_callable instead of method_exists

To use objects implementing __call().

Closes #18.

zuluf/mustache.php,bobthecow/mustache.php,irontec/mustache,evdevgit/mustache.php,BrunoDeBarros/mustache.php,sophatvathana/mustache.php,thecocce/mustache.php,ericariyanto/mustache.php,cgvarela/mustache.php