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.
So like this?

```<% unless node['squid']['enable_ldap'] %>```