Add `sendFile` action

It works better then `sendKeys` for `input[type=file]` elements.
Internally, it uses `element.sendKeys` instead of `driver.type(element)`
which can upload local file to the server.

/сс @scf2k @j0tunn 
```
return isFile
    ? _this._findElement(element)
    : q.reject(new StateError(path + ' should be existing file'));
```