[lint-jss] warn about silly mistakes

When I use the API like this:

``` js
var jss = require('jss');
var sheet = jss.createStyleSheet({
'.foo': {
bar: 0
}
});
console.log(sheet.toString());
```

...then my initial expectation was to get:

``` css
.foo {
bar: 0;
}
```

...but instead, I get:

``` css
..foo-1028573841 {
bar: 0;
}
```

I now know that I need to pass `{named: false}` to the `createStyleSheet` to generate the correct class name.

The same issue occurs when one forgets to use a plugin, e.g. if `jss-camel-case` is not used, then camel-cased properties will be generated without so much as a warning.

Another case of this occurs with `jss-nested`; coming from SCSS, I initially expected this:

``` js
var jss = require('jss');
var nested = require('jss-nested').default;
jss.use(nested());
var sheet = jss.createStyleSheet({
'foo': {
'> *:first-child': {
'bar-bar': 0
}
}
});
console.log(sheet.toString());
```

...to generate:

``` css
.foo-3691883159 > *:first-child {
bar-bar: 0;
}
```

...but instead, it generates:

``` css
.foo-2511288833 {
> *:first-child: [object Object];
}
```

I should have used the syntax `& > *:first-child` (with the `&`). The ampersand was implied in SCSS, but omitting it in JSS resulted in strange output.

It seems that non-sensical class name generation can be a "gotcha" for those new to JSS.

I think it'd be helpful if JSS could detect if it's generating malformed CSS. Maybe this functionality could be available as a development-only plugin, so performance wouldn't need to suffer. It could be as simple as detecting the use of basic CSS syntax / weird casing in keys and printing warnings, or as thorough as a "JSS Lint" utility with knowledge of all valid property names. What do you think?