added partial matching and case insensitivity of searchable values

spec/lib/searchable_spec.rb
added specs to test partial matchers
added specs to test case sensitivity

lib/searchable/is_searchable.rb
refactored #find_by_value params to default associations to self preventing superfluous ternary operators
removed the item variable and replaced it with associations due to refactoring
updated the associations#where query to the 'LIKE' operator