Removed PreloadReader / CachingReader

The preload connection string now throws an unsupported
exception and refers to our wiki for explanations.

Fixes #312
Merged ForwardsOnlyDataReader into base class

Merged ForwardsOnlyDataReader into the abstract base
class NpgsqlDataReader. We now have a single reader
called NpgsqlDataReader.
Cosmetic cleanup of NpgsqlDataReader

Grouped methods in #regions, added some auto-properties. No actual
code change.
Merge branch 'kill_preload'