Updated documentation, Sundown and minitest.

 - Added a new page to the documentation about development.
 - Updated changelogs and THANKS file.
 - Bumped version numbers.
 - Updated Sundown.
 - Updated minitest.
Revert base_url.
Change failing test.
Updated Sundown files.
Update description of update_vendor command.
SmartyPants is now a preprocessor instead of a postprocessor.
Updated documentation.
Revert change to SmartyPants.
Minor addition to development manual.
Update extension flag in script/misaka.
Added and test and updated an extension flag.
Add test command to setup.py.
Fix extra HTML flags.
Update Sundown.
Change submodule repository.
Add more environments to Travis CI conf.
Update paths to Sundown repository.
Added fixed small issue in docs.
Make subclassing easier by using __init__ instead of __cinit__ in BaseRenderer.
머지하고 서브모듈 관리 변경

* misaka 최신 커밋까지 머지
* `init.sh`를 제거하고 서브모듈과 symlink로 연결