Implement searcher simply
Define EPUB::Searcher::Result module for exntensibility
Add converter methods for XPath and CFI
Add current node to returned value of #to_xpath_and_offset
Make search result triple of parent, start and end
Remove unceccessary blank line
Fix alignment
[BUG FIX]CFI includes text step
Add descriptions of arguments for Result#initialize
Change method name: to_cfi -> to_cfi_s
[REFACTORING]Define Result::Step#to_cfi_s and call it from Result#to_cfi_s
Move utility methods for TestSearcher to parent class
Add Searcher::Publiction
Change searcher class name: XHTML -> XHTML::Restricted
Make Searcher a module
Add document about Searcher