Migration Driver for SQLite

Need more tests.
Integrate MigrationDriver to migrate.go
Fix migration step not checked against current version.

Add tests for DropColumns.
More tests for alter columns migration

Also change the way we handle columns rename migration.
SQLite restrict column addition not to have PRIMARY KEY and/or
UNIQUE attribute, so we have to change from:

add new column -> migrate data from old column to new column ->
rename old table -> create new table with old columns removed ->
migrate data from old table to the new table -> drop old table

to directly:
rename old table -> create new table with renamed columns ->
migrate data from old table to the new table -> drop old table
Add tests to migrate existing database without migration attributes.
Integrate migrations with drone.

Also add migration to tests.
Merge pull request #83 from fudanchii/migration

Migration system for sqlite