Require MysqlPtOscAdapter in PtOscMigration

- it uses the class in is_a? checks, so make sure the class is loaded
Compatibility with activerecord-import 0.5.0 and later

- uses functionality added in zdennis/activerecord-import#120 to provide plugin-compatible adapter
- added test
Fix typo in README
Test against Ruby 1.9.2
Added compatibility section in README

- mention activerecord-import 0.5.0 compatibility
Bump version to 0.1.0

- added CHANGELOG entry
Merge pull request #6 from steverice/activerecord-import-compatibility

activerecord-import compatibility