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)```
Merge branch 'master' into quickcast
Merge branch 'master' into quickcast

Conflicts:
	lib/dialects/abstract/query-generator.js
Merge branch 'quickcast' of https://github.com/durango/sequelize into durango-quickcast
cast+literal
Merge branch 'master' into milestones/2.0.0