Allow to open file by current word

This is for #12 issue.
Usage: put cursor on class name (for example "User") and press 'alt-shift-R'. Atom rails will open first file that contains line "class User".

Currently works fine with one level class or module definitions. It will not work if you try to open by work "Admin::User" and you have defined user class as:
```ruby
module Admin
  class User
  # ...
  end
end
```

but it should work fine if you defined your user class as:
```ruby
class Admin::User
  # ...
end
```

P.S. I am ashamed that I did not add any tests. I will try to fix this in the future