Some classes are missing after converting mixins.less into mixins.styl

In less, a mixin like below:
```
.clearfix {
  clear: both;
}
```
is also compiled into a class.

There are three mixins like this in bootstrap 2.3.2:
```
.clearfix
.hide-text
.input-block-level
```
They are all compiled into a class in the final bootstrap.css.

But those three classes are missing in bootstrap-stylus. This pull request is to add equivalent classes for those three mixins.