Only convert single-expression methods to attrs

Should fix issues in #15

and also this one:

```ruby
def b
  @b
  @a
end
```

```
$ r2r_show test.rb
malloc_limit=50000000 (8000000)
heap_min_slots=500000 (10000)
/Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/gems/sexp_processor-4.2.0/lib/sexp.rb:173:in `find_node': multiple nodes for ivar were found in s(:b, s(:args), s(:ivar, :@b), s(:ivar, :@a)) (NoMethodError)
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/gems/sexp_processor-4.2.0/lib/sexp.rb:209:in `method_missing'
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/gems/ruby2ruby-2.0.4/lib/ruby2ruby.rb:322:in `process_defn'
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/gems/sexp_processor-4.2.0/lib/sexp_processor.rb:218:in `block (2 levels) in process'
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/gems/sexp_processor-4.2.0/lib/sexp_processor.rb:275:in `error_handler'
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/gems/sexp_processor-4.2.0/lib/sexp_processor.rb:217:in `block in process'
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/gems/sexp_processor-4.2.0/lib/sexp_processor.rb:340:in `in_context'
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/gems/sexp_processor-4.2.0/lib/sexp_processor.rb:194:in `process'
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/gems/ruby2ruby-2.0.4/bin/r2r_show:31:in `block in <top (required)>'
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/gems/ruby2ruby-2.0.4/bin/r2r_show:24:in `each'
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/gems/ruby2ruby-2.0.4/bin/r2r_show:24:in `<top (required)>'
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/bin/r2r_show:23:in `load'
	from /Users/collins/.rvm/gems/ruby-1.9.3-p392@WHATEVS/bin/r2r_show:23:in `<main>'
```