Done ex1.21
Done ex1.22
Modify 1.22
Add 1.23 and 1.24
Done ex1.27.scm
Change file format from scm to rst
Merge branch 'master' of github.com:sicp-reading/sicp-answers
Merge branch 'master' of github.com:sicp-reading/sicp-answers
Update ex1.16.rst
Update ex1.17.rst
Update ex1.18.rst
Modify .gitignore
scm to rst
Merge branch 'iriya-ufo-scm-to-rst'
Merge