Do not inject self in IPython namespace

And do not require namespace either

  - this is bad practice, and this prevent from
    writing a blended content manager that works both
    with drive and localhost as they should both export
    themselves to `IPython.Contents` module.


If we really want `IPython.Contents` then the various `main.js` should set `IPython.Contents` to the `Contents` they import. 

Though it is unnecessary as one can still do `C = require('contents')` for the global content manager or even `var c = require('explicit/module/path')`
+1 for this style of require, as it is more consistent with npm and browserify projects.  I too think it's easier to read.