Bitstring, "char" type, parameter fixes.

This is based on pull request #362 and implements the following:

* BitString are now correctly returned from backend, and is correctly handled in parameters.
* BIT(1) maps to the .NET bool type and other BIT maps to BitString.
* Output parameters now work correctly for prepared queries.
* DbType.Time is now mapped to interval.
* The "char" type now returns a Char instead of a String.
* Type OIDs are sent in the Parse message.
* Optimizations when all format codes are the same in Bind message.
* Automatically infer parameter value types when setting a value.
* New NpgsqlDbType.Unknown that specifies that Npgsql should not try infer the value type, nor cast the value when written to sql, but simply write a literal string in text form.
OK, that makes sense. But there's another (theoretical) possibility: the user *did* specify a parameter type but the backend returns a different one. I'm not sure if/when this can happen. So it may be good to check for the case you described (no type specified), and otherwise to do an assertion failure..