refactored some code and all specs still pass

README.md
updated installation walkthrough to use 'active_search' instead of 'searchable'
reworded some areas for easier reading

lib/active_search/is_searchable.rb
fixed indentation issues of @searchable_values to ensure clairity of assignment location

lib/active_search/search_helper.rb
removed the 'association' parameter of ActiveSearch::IsSearchable::ClassMethods#find_by_value because it will never actually receive an association since an association is only called on the instance of a class.
Merge pull request #19 from zachmokahn/07-refactoring

refactored some code and all specs still pass