Add support for connecting to a DB2 catalog name

In our environment, we use catalogs extensively on the clients to define the location of a server.  (Our DBA can then shift servers around without the clients knowing about it)

This patch allows you to specify just the `:database` parameter along with the `:user` and `:password` to connect to a DB2 server.  If a `:host` and `:port` are specified, it falls back to the ODBC string definition.

```ruby:
# Explicitly connect to a host and port
db = Sequel.connect(:adapter => 'ibmdb', :host => 'server', :port => 50000, 
                    :database => 'prod', :user => 'username', :password => 'pass')
# Use a locally defined DB2 catalog
db = Sequel.connect(:adapter => 'ibmdb', :database => 'dev', 
                    :user => 'username', :password => 'pass)
```