Remove unnecessary virtualenv options

`--distribute` is now a no-op since the merge of distribute back into setuptools.
`--no-site-packages` is now the default.
Merge pull request #134 from ghickman/patch-1

Remove unnecessary virtualenv options
Regenerated stdlib.go
Add a python3 layout

Create a virtualenv with the system version of Python 3.

This picks a user's `python3` from the path which may not be the correct version but provides a simple base.
Fix python3 function name
Merge pull request #135 from ghickman/patch-1

Add a python3 layout
Add -g option.
Merge pull request #139 from mashiro/add-global-option

Add -g option.
stdlib warning fixes using shellcheck

see https://github.com/koalaman/shellcheck
Allows to specify which version of python to use in layout_python
Makes `layout python3` a shortcut for `layout python python3`
Use a different virtualenv per python versions

This changes the path of the virtualenv folder. The old path is detected and
re-used to avoid unecessary re-installation.
Merge pull request #136 from zimbatm/python-layout-option

Python layout option
Remove unnecessary virtualenv options

`--distribute` is now a no-op since the merge of distribute back into setuptools.
`--no-site-packages` is now the default.
Regenerated stdlib.go
Add a python3 layout

Create a virtualenv with the system version of Python 3.

This picks a user's `python3` from the path which may not be the correct version but provides a simple base.
Fix python3 function name
Add -g option.
stdlib warning fixes using shellcheck

see https://github.com/koalaman/shellcheck
Allows to specify which version of python to use in layout_python