support standard markdown syntax, lheading and hr tag

# support standard markdown syntax (setext)

## test code

```
# Standard Markdown Rendering

Title
-

- list1
 - list1
 - list 2
- list2

-

--test

-test

Title
=

```

## github rendering

![923fdbd8-fca7-11e2-9217-282211662b80](https://f.cloud.github.com/assets/1558742/907935/30468cf0-fd70-11e2-95e9-dbab82e78920.png)

## markdown editor rendering (on mac osx)

![a662263e-fca7-11e2-830d-7e2942beb00c](https://f.cloud.github.com/assets/1558742/907936/3fa0a988-fd70-11e2-9777-afafe3a6f441.png)

## currently marked rendering

![f3c26c2c-fca7-11e2-841e-8c03723055d8](https://f.cloud.github.com/assets/1558742/907937/5102116c-fd70-11e2-8d13-a761cce41bf0.png)

## after fixed 

![2d568914-fca8-11e2-995c-f958085ae084](https://f.cloud.github.com/assets/1558742/907939/5e1fa5b2-fd70-11e2-866a-2b2593465bf4.png)