TypeHandler<> can't override default typeMap for base types

[maurodx]

Trying to solve an issue (losing milliseconds on DateTime in SQLite database) using a TypeHandler that do conversion between db string (in format "o") and DateTime doesn't pass test because typeMap[DateTime] is processed before custom type handlers.

Probably the solution is to invert the lookup sequence or add the chance for remove a default type map with a brother of

public static void AddTypeMap(Type type, DbType dbType)

as

public static void RemoveTypeMap(Type type)

[maurodx]

By now I have to use Reflection in order to remove from private typeMap the defaults for DateTime and DateTimeOffset.. Not a great solution but only a workaround

[maurodx]

Probably can be solved via pull request #177

[NickCraver]

See #471 up for v2, closing out to consolidate.