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.
By using the parameters collection to populate the parameterTypeOids in a 1-1 mapping, you can get a problem when the same parameter is used more than one time in the query string. For example, in the following query: 
select * from table where column1= :param1 and column2= :param1