Prevents MySQL driver to failing when version table alredy exists

The MySQL driver is using `CREATE TABLE IF NOT EXISTS`, which causes
the MySQL database to raise warnings. The golang driver collects
the warnings and return a composite object (mysql.MySQLWarnings) as
an error that needs to be properly handled.

This change stops the driver from failing in case there are only
warnings.
Don't consider warnings as errors
Rewrite import paths
Don't stop when a warning happens
Don't stop when a mysql warning happens
Merge branch 'mysql_version_table_issue'
Merge branch 'master' of github.com:divoxx/migrate