NpgsqlParameter

Francisco,

This PR aims to move parameter type coercion to NpgsqlParameter, so that type coercions can be done as parameter values, types, and modifiers are set, rather than when they are encoded for transmission to the backend.  I believe that once this is done, we'll be able to improve prepared statements because we'll have all type information set up before the Parse command is executed.

This is a work in progress.  I started it about a month and a half ago, and haven't looked at it in at least a month.  Many tests fail, and lots of commented code is still floating around. With PR #77 merged, it became reasonable to submit this one for early review.  Comments welcome!

-Glen