Update _st.js

Added description of _st function. See email exchange with Herby Fri, Sep 6, 2013 at 11:28 AM
Merge pull request #656 from hhzl/master

Update _st.js

Merging myself (it does not make any functionality change anyway), I want to slightly refactor this so I want it in.
_st defined in directly in its module not in boot.js
Fixes #627
The same version of jquery-ui in legacy and helios IDE
defaultAMDNamespace -> defaultAmdNamespace
Merge pull request #659 from herby/gh-627

Fixes #627
Merge pull request #660 from herby/same-jquery-ui

The same version of jquery-ui in legacy and helios IDE
Merge pull request #661 from herby/Amd-AMD-inconsistency-fix

defaultAMDNamespace -> defaultAmdNamespace
Merge pull request #657 from herby/st-out-of-bootjs

_st defined directly in its module not in boot.js
names of low-level modules always explicit in define
added `amber/helpers`, you can popup Helios conveniently

Use `require('amber/helpers').popupHelios()`.
Merge pull request #663 from herby/helpers-in-popup-helios-possible

added `amber/helpers`, you can popup Helios conveniently
Merge pull request #662 from herby/names-in-define

names of low-level modules always explicit in define