Update linters. Closes #557

* Update git-hooks and jscs versions
* Update jshint-groups and jscs configs
* Check bemhtml and bemtree with jscs
* Fix all errors

// cc @narqo @dfilatov 
`(ctx.content == null) === (typeof ctx.content === 'undefined' || ctx.content === null)`

Don't really think that `content` could be exact `null` in general, but it helps to filter out other "falsy" values like `0`, `false` etc:

```js
nul == null  // true
nul == undefined  // true
nul == 0  // false
nul == false  // false
```