YML error escaping @ as initial value's char

If char '@' is first character of a yml value escaping fail.

Example:

``` YML
parameters:
    ad_ldap:
       account_suffix: mydomain.com
       base_dn: "@DC=mydomain.com"
       domain_controllers: 
          - 100.100.176.123
       real_primarygroup: true
```

I thinks it's related to #173
