Allow to open file by current word

[povilasjurcys]

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:

module Admin
  class User
  # ...
  end
end

but it should work fine if you defined your user class as:

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