Clean up tree_dresser.rb

- Move SexpExtensions and SexpNode into their own files
- Cleaner extension lookup
- Fix code smells
Simplify implementation of DataClump

- Use Enumberable#each_cons to find all sets of methods to check
- Add and improve specs
Merge pull request #212 from troessner/issue-fix-data-clump

Simplify implementation of DataClump
Merge pull request #218 from troessner/issue-clean-up-treedresser

Clean up tree_dresser.rb
Refactor DuplicateMethodCall smell detector
Merge pull request #219 from troessner/issue-refactor-duplicate-method-call

Refactor DuplicateMethodCall smell detector