Added support for simple enums in schemas

Adds support for the simple enum syntax: https://github.com/LearnBoost/mongoose/issues/1525
Because mongoose supports specifying types as strings it only applies if the array has more than one item (which is generally the case for enums anyway)

Example:
`````
var user = new Schema({
    name: String
    role: ['admin', 'moderator', 'editor', 'user']
})
````

I've added the tests for it as well