Fix switch to ipython console when importing rootpy.interactive

Delay the instantiation of InteractiveShellEmbed until
first usage.
Merge pull request #260 from apuignav/rootpy-issue-259

Fix switch to ipython console when importing rootpy.interactive
Updated label position
Temporary fix for interactive loading ipython
Reverted back the fix
Merge branch 'master' of github.com:apuignav/rootpy

* 'master' of github.com:apuignav/rootpy:
  Temporary fix for interactive loading ipython
  Updated label position