Adds sequelize.literal() and sequelize.cast()

sequelize.literal() will tell Sequelize to not escape whatever you type
into the argument, for example:

sequelize.literal('1-2') will result in "1-2" unescaped.

sequelize.cast(input, type) will cast input into type, for example:

sequelize.cast('1-2', 'integer') will result in:

```CAST('1-2' AS INTEGER)```