Update create_connect_args with database argument

[sandan]

The dialect should include an argument in create_connect_args for specifying a default database to connect to: "teradata://user:pass@hostname/database".

Since this dialect relies on the teradata/tdodbc.py module, a PR has been opened against that project:
Teradata/PyTd#57

[sandan]

The database parameter gets passed in a dictionary from cparams already in the dialect. This should allow the database parameter, if given, to be passed as a kwarg to the tdodbc module. The database is specified in the connect string to the engine like so:
'teradata://user:pass@localhost/database'