introduce SqlResource notion.

Until now, 2way-sql resource was represented as String which indicates
classpath resource.  This commit introcude SqlResource interface as
more abstruct notion of 2way-sql resource.

Users can use various sql resource representation such as URL, local
filesystem and in memory String.
add deprecation comments.
introduce StringSqlResource implementation.

This SqlResournce implementation represents SQL as a String.
So StringSqlResource implements SqlReource interface, you can use this
resources instead of SqlManager#...BySql(..., String sql, ...) methods.

Introdution this abstruction, there is no requirements of
SqlManager#...BySql methods.
Merge pull request #14 from dai0304/resource_representation

New resource representation: SqlResource