Optional array argument of table names to skip

dropAllTables now accepts optional array argument of tables names
to skip dropping these tables

implementation for sequelize/sequelize#1270
dropAllTables now accepts an options object

If this object contains a propert called `skip` that is an array,
then all table names in this array wont be dropped
use triple equals
Merge pull request #1280 from bulkan/feature/skip_tables_dropAllTables

Optional array argument of table names to skip