Adding LDAP authentication

Adding LDAP authentication to the squid cookbook.  I've tested this internally against my LDAP server and it works.  The really long line in squid.conf.erb is a little ugly.  Maybe there is a better way to write that?

It should not apply any of these changes unless the node['squid']['ldap_host'] attribute is specified.

The two additional data bag items are also required.  I hope I made that clear enough in the README.
I'd actually prefer to have a separate attribute as a flag, rather than depending on this being set. something like `node['squid']['enable_ldap']` where the default is `false`