Use semver directory to carry the schema version

[schnell18]

Hi, This PR is an attempt to use semver directory to represent the schema version. The current version scheme is not intuitive and requires additional efforts to keep track of the database version with the application version.

This PR moves the version component from the file name to its containing directory. The name of directory conforms to the semver standard. Currently, this feature is available to the file source.

In addition, there are two extra fields added to the schema_migrations table so that we can track when is the schema change applied last time as well as the change identifier w/ semver version number embedded.

Please consider merge this PR or feel free to get back to me for further improvement. Thanks for your attention!

[coveralls]

coverage: 59.809% (+1.3%) from 58.526% when pulling f66f4eda37896eb8aa5a6090e7f6c3d742d37203 on schnell18:master into 23d8d33f5743c7a9f7bfb3f7bc2043f7e9348e7c on golang-migrate:master.