Drop dependency on dblog module

Hi,

We run with syslog for watchdog logging in a cluster, with dblog disabled (keeps our database at a sane size). We want to use your module to warn us of certain events that need immediate attention, but there's a dependency on dblog.

It looks like the only reason there's a dep is to get at the _dblog_get_message_types() function for looking up all the types of messages available. Given that we don't have any messages in the database anyway, that wouldn't work even if we did have that function available.

I've crafted a small patch that allows the module to still work without dblog, but will require the type field to be filled in as a text field rather than a select box when dblog isn't installed. Works for me!