Add function to toggle between current spec/method and method/spec

[better specs](http://betterspecs.org/) advise you to describe methods in the following manner:

given:
```ruby
def methodz(args)
  #do some stuff
end

def self.klass_methodz
  #do some class stuff
end
```

we should have in the spec:
```ruby
describe '#methodz' do
   #examples here
end

describe '.klass_methodz' do
  #other examples
end
```

I have added a function that jumps between the method definition and it's description and vice versa. Both instance and class methods are covered.

I have also added keybindings.
Wow, I could swear this definition is local. I've read a little bit about `cl-labels` and it seems like the right thing to use.