tweak some python's snippets

- in python2, class declaration should inherit other classes or object.
- rewrite a file idiom under the with statement
- fix with-snippets bound by open function
Add codeblock snippet to markdown.snip
Merge pull request #7 from pocke/for_markdown

Add codeblock snippet to markdown.snip
Merge pull request #6 from hachibeeDI/python_snip

tweak some python's snippets
Improve assert snippet
tex: add online snippet.
Merge remote-tracking branch 'upstream/master'