Support bundler

Use 'bundler exec rake' if there is '.bundle/' on project top directory.
Update README.md
add test case
refactor extract_symbols to extract some methods which have no arugments
Check 'Gemfile.lock' instead of '.bundle'

See also
  - http://gembundler.com/v1.3/rationale.html
Merge pull request #9 from syohex/support-bundler

Support bundler
Implement completing rake tasks

To complete rake tasks is slow, but it can reduce mistakes.
Fix for setup history
Merge pull request #11 from syohex/rake-task-completion

Implement rake tasks completion
hack to be enable asynchronous caching rake task list.