add how to connect mysql via unix socket

the pdo library is support dsn options, but the document didn't have any message about that.
some people may need to connect mysql via unix socket. so i add the dsn and usage, maybe it will help more people.
Update Phalcon_Db_Adapter_Pdo.rst
Merge pull request #419 from Barbery/patch-1

add how to connect mysql via unix socket